The same colour on the same material can look like several colours. Which one you see depends on three things besides the pigment: how smooth the surface is, what kind of light falls on it and from where, and where you stand. These tables set out 29 combinations, each with the reason it happens, the form in which it turns up as a problem, and what to do about it.
Every row gives the direction of an effect and its cause. None gives a number, because how large the effect is depends on the product, the lamp and the room, and no source measures that in general. A combination that could not be sourced is left out, so the tables are deliberately incomplete.
To see the first of these on a colour of your own, use the finish simulator, which models matt, satin and gloss for a single colour. This page covers what the simulator leaves out: the light, the angle and the material.
Applies to: Painted and coated surfaces, Laminates and engineered stone, Plastics, Fibre-reinforced composites (gelcoated GRP), Vitreous (porcelain) enamel, Plaster (lime and gypsum).
| Surface and finish | Light | Viewed | What changes | Why | How it goes wrong | What to do | Evidence |
|---|---|---|---|---|---|---|---|
| Dark gloss paint, lacquer, laminate or moulded plasticFinish: Gloss | Single small source (spotlight, downlight, bare lamp) | At the mirror angle of the light | A bright image of the lamp replaces the colour at that spot; the darker the colour, the stronger the contrast between the hot spot and the surface around it. | Mirror-like (Fresnel) surface reflection. A smooth surface sends its surface reflection in one mirror direction. That reflection is light from the source, not from the pigment, so at the mirror angle it is added on top of the colour and swamps it.More: Gloss and colour, SCI vs SCE | A dark gloss door or run of cabinets shows every downlight as a hot spot, and the spots move as you walk. | Move or diffuse the lamps so their mirror angle misses the usual viewpoint, or choose a satin or matt finish for dark colours under spotlights. | Strong evidence3 sources. How objectionable the reflection is depends on the size and brightness of the lamp and on the room behind the viewer; none of that is quantified here. |
| Dark gloss paint, lacquer, laminate or moulded plasticFinish: Gloss | Single small source (spotlight, downlight, bare lamp) | Face-on, away from any reflection | Away from the reflection the colour looks darker and more saturated than the same colour in a matt finish. | Mirror-like (Fresnel) surface reflection. Because the surface reflection is confined to the mirror direction, almost none of it reaches a viewer anywhere else, so the pigment colour arrives without a veil of source-coloured light over it.More: Gloss and colour | A gloss sample chosen for its depth looks chalkier and lighter when the same colour is ordered in matt. | Compare samples in the finish that will be used, held at the same angle and away from reflections. | Strong evidence3 sources. The direction is well established; its size depends on the colour, the gloss level and the lighting, and is larger for dark colours than pale ones. |
| Matt paint or any matt coatingFinish: Matt | Any light | Any angle | Looks lighter and less saturated than the same colour in gloss, and changes little as the viewpoint moves. | Surface reflection scattered by a rough surface. A rough surface scatters its surface reflection in every direction instead of into one mirror direction, so some uncoloured light is added to the colour wherever the viewer stands.More: Gloss and colour, Texture and colour | A deep colour in flat paint reads dustier than expected, and a matt black never looks as black as a gloss one. | Accept the lift as the price of an even, reflection-free wall, or step up to eggshell or satin where a dark colour needs depth. | Strong evidence2 sources. Pale colours are affected far less than dark ones, because a little added white is small beside the light they already reflect. |
| Gloss or semi-gloss paint, lacquer or laminateFinish: Gloss | Diffuse or large-area light (overcast sky, big window, lit ceiling) | From an angle | A large pale reflection of the window, sky or lit ceiling lies over part of the surface, and the colour there looks lighter and washed out. | Mirror-like (Fresnel) surface reflection. The mirror reflection is an image of whatever sits at the mirror angle. A big bright source gives a big reflected image, so a wide patch of the surface carries source light on top of its colour.More: Gloss and colour | A gloss wall opposite a window shows the window, and looks like two colours — one inside the reflection, one outside it. | Keep gloss for surfaces that do not face large windows at the mirror angle, or lower the sheen; blinds change the reflection, not the paint. | Moderate evidence2 sources. Follows from mirror reflection; no source measured how large or how pale the reflected patch is for a given room. |
| Matt, eggshell or satin paint and other low-sheen coatingsFinish: Satin or eggshell | Any light | Along the surface (grazing) | Seen along its length, a low-sheen surface shows a sheen it does not show face-on, and looks paler there. | Mirror-like (Fresnel) surface reflection. Surface reflection strengthens as the angle of view approaches the surface, which is why low-gloss finishes are measured with a glossmeter set close to grazing rather than at the usual angle.More: Gloss and colour | A wall that looks evenly matt from the middle of the room shines when seen down a corridor or beside a window. | Judge sheen by sighting along the sample as well as facing it, and choose a flatter finish for long walls seen end-on. | Moderate evidence3 sources. Sheen names such as eggshell and satin are not standardised between manufacturers, so the grazing sheen of two products with the same name can differ. |
| Two painted surfaces, or paint beside another material, matched by eyeFinish: Any finish | Two or more different sources | Any angle | A pair that matches under one light can separate under another, and the direction of the mismatch depends on the two formulations. | Metamerism. Two surfaces can look alike under one light while reflecting different spectra; a light with a different spectrum then draws different responses from each.More: Illuminant metamerism, Why colours match in the shop but not at home | Touch-up paint, or a door and its frame from different products, match in daylight and clash under the evening lamps. | Check matches under every light the surfaces will be seen in, and touch up from the original tin rather than a re-mixed one. | Strong evidence2 sources. Two coats of the same paint from the same batch have the same spectrum and cannot be metameric; a mismatch between them has another cause, such as sheen or film thickness. |
| White and near-white matt paintFinish: Matt | Two or more different sources | Any angle | Takes on the colour of whichever light dominates each part of the room: warmer near a warm lamp, cooler near a window. | The spectrum of the light source. A white surface reflects all visible wavelengths strongly, so the light it returns has nearly the spectrum of the light that reaches it; warm and cool sources differ greatly in how their power is spread across the spectrum.More: Spectral power distribution, explained, Colour constancy and chromatic adaptation | One white wall looks cream beside the lamp and blue-grey beside the window, and is blamed on the paint. | Judge whites in the room's own light at the times it is used; bring the lamps closer to one colour temperature if the split is unwelcome. | Moderate evidence2 sources. The eye adapts to the prevailing light, which hides much of the shift when only one source is present; it shows most where two sources can be compared side by side. |
Applies to: Plaster (lime and gypsum), Painted and coated surfaces, Fired clay brick, Concrete, Natural stone (granite, limestone, sandstone, slate).
| Surface and finish | Light | Viewed | What changes | Why | How it goes wrong | What to do | Evidence |
|---|---|---|---|---|---|---|---|
| Painted plaster or plasterboard meant to be flatFinish: Satin or eggshell | Raking light (along the surface) | Any angle | Joints, fixings, ridges and filled patches appear as bands of shadow and highlight that are invisible in frontal light; sheen and dark colours make them stronger. | Shadowing and highlighting of surface texture. Light arriving along the surface brightens any slope that faces it and shadows any slope that faces away, so height differences far too small to notice face-on become visible as contrast.More: Texture and colour, Plaster colour | A new wall beside a tall window, or under a wall-washing light, shows every board joint once it is painted. | Ask for a full skim coat on walls that take side light or will carry sheen or dark colours; use flat paint there; move wall-washers away from the wall. | Strong evidence3 sources. Industry guidance says the result cannot be guaranteed even with the highest level of finish; it reduces the effect rather than removing it. |
| Brick, rough render, textured plaster, board-marked concrete, riven stoneFinish: Textured | Raking light (along the surface) | Any angle | Texture reads at full strength: lit faces brighten, hollows fall into shadow, and the wall looks more contrasty and more shadowed than under frontal light. | Shadowing and highlighting of surface texture. A rough surface is a field of small tilted facets. Under directional light the facets turned towards the source are bright and the others are shadowed or hidden by their neighbours, so the surface departs from the even look of an ideal matt one.More: Texture and colour, Brick colour | A brick or rough-plaster sample approved under the showroom's overhead light looks harsher and more uneven on a wall the low sun runs along. | View a sample panel on site at the hours the sun or the lighting rakes it, and choose the texture for that light. | Strong evidence2 sources. The direction of the lightness change depends on where the viewer stands relative to the light; seen from the side the light comes from, a rough surface can look brighter, not darker. |
| Brick, rough render, textured plaster, board-marked concrete, riven stoneFinish: Textured | Diffuse or large-area light (overcast sky, big window, lit ceiling) | Any angle | Texture flattens: with light arriving from all sides few facets are in shadow, so the surface looks more even and its colour more uniform. | Shadowing and highlighting of surface texture. The shadowing that makes texture visible needs light from one direction. Under a large diffuse source each facet is lit from many directions at once and the contrast between facets falls.More: Texture and colour | A feature wall chosen for its texture looks flat on an overcast day or under a uniformly lit ceiling. | If the texture is the point, give the wall a directional light of its own rather than relying on ambient light. | Moderate evidence2 sources. The converse of the raking-light case, drawn from the same shadowing mechanism rather than from a separate study of diffuse light on masonry. |
Applies to: Wood (solid timber and veneer).
| Surface and finish | Light | Viewed | What changes | Why | How it goes wrong | What to do | Evidence |
|---|---|---|---|---|---|---|---|
| Warm-toned timber and veneer (oak, cherry, walnut, pine)Finish: Any finish | Warm, low colour-temperature source | Any angle | Reads warmer — more orange and red — than it does in daylight. | The spectrum of the light source. A low colour-temperature source puts a much larger share of its power at long wavelengths than daylight does, and wood, whose lignin absorbs at the short-wavelength end, returns mostly that long-wavelength light.More: Wood colour: solid timber and veneer, Spectral power distribution, explained | A floor or worktop chosen in a daylit showroom looks markedly more orange under warm evening lighting at home. | Take the sample home and view it under the room's own lamps in the evening as well as by day before committing. | Moderate evidence2 sources. Colourwise's reading of two cited facts — the spectrum of warm sources and what wood absorbs — not a measured study of timber under lamps. Adaptation of the eye to warm light offsets part of the shift. |
| Warm-toned timber and veneer (oak, cherry, walnut, pine)Finish: Any finish | Cool, high colour-temperature source or daylight | Any angle | Reads cooler and less red than under warm lamps. | The spectrum of the light source. Daylight and cool sources spread their power more evenly across the spectrum, with a smaller share at the long wavelengths that wood reflects most.More: Wood colour: solid timber and veneer, How daylight varies, and how the CIE models it | Timber matched to a paint or fabric under warm shop lighting looks duller and greyer beside a north-facing window. | Match wood to its neighbours under both the daylight and the lamps of the room. | Moderate evidence2 sources. Same basis and limits as the warm-light case: an inference from the spectra of the sources, with no magnitude attached. |
Applies to: Cotton and linen (cellulose fibres), Wool and silk (protein fibres), Polyester and other synthetic fibres, Leather, Paper and card.
| Surface and finish | Light | Viewed | What changes | Why | How it goes wrong | What to do | Evidence |
|---|---|---|---|---|---|---|---|
| Strongly coloured textiles, above all saturated redsFinish: Any finish | Low colour-rendering or narrow-band source | Any angle | Saturated reds look duller and browner under a source that renders red poorly; a source that boosts red makes them more vivid than in daylight. | The spectrum of the light source. A saturated red reflects mainly at long wavelengths. A source short of power there gives the fabric less light to return. The general colour rendering index does not include a strong red sample, so a lamp can score well and still do this.More: R9 and saturated reds, Why fabric changes colour under a lamp | A red sofa, rug or dress looks rich in the shop and flat at home under lamps with a respectable colour rendering index. | Look for a stated R9 or a TM-30 red-fidelity figure as well as the general index, and view the fabric under the lamp type you will use. | Moderate evidence3 sources. The direction comes from Colourwise's calculation for one synthetic red surface under CIE illuminants, described in the lighting section; it is not a measurement of a particular fabric or lamp. |
| Dyed textiles matched to each other or to paint, plastic or leatherFinish: Any finish | Two or more different sources | Any angle | Pieces that match under one light part company under another; fabrics of different fibres are especially prone, because each fibre takes a different class of dye. | Metamerism. Different dyes reach the same colour with different reflectance spectra. A change of light changes each spectrum's result differently, so the match holds only under the light it was made in.More: Illuminant metamerism, Why matching blacks stop matching outside | Curtains and cushions, or a jacket and trousers bought separately, match in the shop and not in the room. | Compare under daylight and under the room's lamps before buying; where an exact match matters, buy pieces cut from the same dyed cloth. | Strong evidence2 sources. Whether a given pair is metameric cannot be told from the fibre labels alone. |
| Whites containing an optical brightener (most white paper, much white fabric)Finish: Matt | Light with ultraviolet in it (daylight) | Any angle | Looks bluer and brighter-white than an unbrightened white beside it. | Fluorescence. The brightener absorbs ultraviolet and re-emits it as blue light, adding to the light the surface reflects and cancelling a yellowish cast. The more ultraviolet in the light, the more it emits.More: Fluorescent colourants and optical brighteners, Two whites clash | White paper, shirts or blinds look almost blue beside a white-painted wall or an unbrightened fabric in daylight. | Compare whites in daylight before combining them; choose materials that are either all brightened or all unbrightened where they meet. | Strong evidence3 sources. Says nothing about whether a particular product contains a brightener; a UV torch in a dark room is the quick check. |
| Whites containing an optical brightener (most white paper, much white fabric)Finish: Matt | Light with little ultraviolet | Any angle | Loses its blue lift and looks creamier, so two whites that matched in daylight no longer match. | Fluorescence. With little ultraviolet to absorb, the brightener emits little, and the material shows the colour of its unbrightened base.More: Fluorescent colourants and optical brighteners, Paper and card colour | A print proofed in daylight looks yellower under gallery lighting, and brightened and unbrightened whites swap places as the light changes through the day. | Assess brightened whites under the light they will live in; for colour-critical print, specify the measurement condition that matches the viewing light. | Strong evidence2 sources. Brighteners also wear out with long exposure to ultraviolet, so an old brightened white behaves like an unbrightened one under any light. |
| Silk and other lustrous woven fabricsFinish: Satin or eggshell | Single small source (spotlight, downlight, bare lamp) | From an angle | Light and dark bands follow the folds, and the same cloth looks a different depth of colour on each face of a drape. | Reflection from aligned flakes or fibres. Smooth fibres with flat faces reflect light directionally, so each fold sends its sheen a different way: folds that mirror the lamp look pale, those that do not show the dye colour.More: Angle-dependent colour, Wool and silk colour | A silk swatch held flat looks like one colour; made up into curtains it looks like several. | Judge lustrous fabrics draped and under the room's own lamps, not flat on a table. | Moderate evidence1 source. Sourced to a reference article on the fibre's shape and sheen; the banding described is its direct consequence, not a separately measured result. |
Applies to: Painted and coated surfaces, Aluminium (anodised, powder-coated, mill).
| Surface and finish | Light | Viewed | What changes | Why | How it goes wrong | What to do | Evidence |
|---|---|---|---|---|---|---|---|
| Metallic paint and powder coat (aluminium flake)Finish: Metallic or pearlescent effect | Direct sun | At the mirror angle of the light | At its lightest and most brilliant near the mirror angle, with individual flakes sparkling. | Reflection from aligned flakes or fibres. Flakes lying roughly parallel to the surface act as small mirrors. Near the mirror angle most of them return the light to the viewer, and under a direct source single flakes flash.More: Angle-dependent colour, Metallic and pearlescent pigments, Solid, metallic, pearlescent and matte car paint | A metallic sample card viewed square-on in the sun looks far lighter than the finished panel seen from across the room. | Approve metallic finishes by tilting a real panel through the angles it will be seen at, not from a flat card. | Strong evidence2 sources. Flake size and how the coating was applied change the effect, so two batches of one colour can differ in it. |
| Metallic paint and powder coat (aluminium flake)Finish: Metallic or pearlescent effect | Direct sun | From an angle | Darkens steadily as the view moves away from the mirror angle — the light-to-dark 'flop' that makes a curved panel read as curved. | Reflection from aligned flakes or fibres. Away from the mirror angle the flakes send their reflection elsewhere, and the viewer is left with the darker colour of the pigmented layer between them.More: Angle-dependent colour, Measurement geometry: 45°/0° and d/8° | A flat metallic wall or run of panels looks patchy, because the viewing angle changes across it; a resprayed panel is light where its neighbour is dark. | Keep strong-flop metallics for curved or small surfaces; have repairs matched at several angles, not one. | Strong evidence2 sources. Industry measures this at several fixed angles; a single-angle reading cannot describe it. |
| Metallic paint and powder coat (aluminium flake)Finish: Metallic or pearlescent effect | Diffuse or large-area light (overcast sky, big window, lit ceiling) | Any angle | Sparkle disappears and a coarser 'salt and pepper' graininess can show instead, with little change as the viewpoint moves. | Reflection from aligned flakes or fibres. Sparkle needs a small bright source for each flake to mirror. Under an overcast sky the flakes mirror an even field of light, and what remains visible is the irregular pattern of the flakes themselves.More: Angle-dependent colour | A metallic finish that glittered in the sunlit showroom looks flat and slightly mottled on a grey day. | Look at effect finishes under both direct and overcast light before choosing; finer flake gives a smoother look in diffuse light. | Strong evidence1 source. An instrument maker's account of visual assessment practice, in line with the standards it cites; the terms sparkle and graininess are its own. |
| Pearlescent and interference finishes, interference-coloured anodisingFinish: Metallic or pearlescent effect | Direct sun | From an angle | Hue and saturation, not just lightness, change as the angle of view or of the light changes. | Thin-film interference. Thin transparent layers on each platelet reinforce some wavelengths and cancel others, and which ones depends on the path the light takes through the layer — that is, on the angle.More: Angle-dependent colour, Interference and structural pigments | A pearlescent colour approved on a small flat chip shifts hue across a large or curved part, and two parts set at different angles never look the same colour. | Approve on a part of the real shape; keep adjacent parts at the same angle; accept the travel as the purpose of the finish. | Strong evidence3 sources. Which hues a finish travels between depends on the pigment's layer thicknesses; no general rule gives the colours. |
Applies to: Natural stone (granite, limestone, sandstone, slate), Marble, Ceramics, glazes and porcelain, Laminates and engineered stone, Vitreous (porcelain) enamel.
| Surface and finish | Light | Viewed | What changes | Why | How it goes wrong | What to do | Evidence |
|---|---|---|---|---|---|---|---|
| Polished stone, polished porcelain and glazed tileFinish: Polished | Single small source (spotlight, downlight, bare lamp) | At the mirror angle of the light | Each lamp appears as a sharp bright image on the surface, hiding the pattern and colour beneath it. | Mirror-like (Fresnel) surface reflection. Polishing or glazing makes the surface smooth enough to reflect like a mirror, so the surface reflection forms an image of the source instead of spreading out.More: Gloss and colour, Natural stone colour: granite, limestone, sandstone and slate | A polished worktop under a row of downlights, or a glazed-tile splashback under cabinet lights, shows a line of glare spots across its pattern. | Use a diffused or linear light set back from the mirror angle, or choose a honed stone or matt glaze. | Moderate evidence3 sources. The mechanism is that of any gloss surface; it was not drawn from a study of stone or tile specifically. |
| The same stone, polished and honedFinish: Polished | Any light | Face-on, away from any reflection | The polished face looks darker and more saturated, with more contrast in its veining, than the honed face of the same slab. | Mirror-like (Fresnel) surface reflection. A honed face scatters its surface reflection in all directions and so veils the stone's colour; a polished face sends that reflection one way and leaves the colour clear from everywhere else.More: Gloss and colour, Marble colour | Honed tiles ordered to match a polished sample of the same stone arrive looking paler and greyer. | Choose from a sample in the finish that will be laid; expect a re-honed or etched patch to look paler than the polish around it. | Moderate evidence2 sources. Stone varies slab to slab, so some of any difference between two samples is the stone and not the finish. |
| Glazed tile, vitreous enamel and polished stone on a wall or floorFinish: Gloss | Diffuse or large-area light (overcast sky, big window, lit ceiling) | From an angle | A pale reflection of the window or sky lies across the surface and the colour within it looks washed out; on a floor the reflection strengthens towards the far end. | Mirror-like (Fresnel) surface reflection. The surface mirrors whatever lies at the mirror angle, and mirror reflection strengthens as the view approaches the surface, so a floor seen at a low angle reflects far more of a window than a wall seen face-on.More: Gloss and colour, Ceramic, porcelain and enamel colour | A dark polished floor running towards a glazed door reads as a sheet of glare rather than as its own colour. | Choose a honed or matt surface for floors that run towards large windows; keep polished finishes for surfaces seen more nearly face-on. | Moderate evidence2 sources. Follows from surface reflection in general; no source measured it for a floor. |
| Marble, alabaster, onyx and other translucent stoneFinish: Translucent | Lit from the front | Any angle | Looks soft and slightly glowing: shadows and carved detail are gentler than on an opaque stone of the same colour, and thin edges look lighter. | Light scattered inside the material. Light enters the stone, scatters inside it and leaves from a different point, so light bleeds into areas that would otherwise be in shadow.More: Translucency and subsurface scattering, Marble colour | A painted or opaque imitation of marble looks hard and flat beside the real stone even when its surface colour matches. | Compare candidate materials under directional light, where the softening shows; a colour match alone will not reproduce it. | Strong evidence2 sources. The primary source is a rendering model built on measurements of marble, not a survey of building stones; how far light travels varies with the stone. |
| Thin slabs of marble, alabaster or onyxFinish: Translucent | Lit from behind | Any angle | The colour is now the colour of the light that gets through: thicker and more absorbing areas look darker, and veining that was faint on the surface can dominate. | Light absorbed on its way through the material. Lit from behind, the slab is seen by transmitted light, which has crossed its whole thickness and been absorbed and scattered along the way; lit from the front, it is seen by light that only visited the outer layer.More: Translucency and subsurface scattering | A slab chosen by its front-lit face looks a different colour, and far more strongly patterned, once the light behind it is switched on. | Approve the actual slab, at its finished thickness, with a light behind it and with that light switched off. | Limited evidence3 sources. The sources establish that thin stone transmits light and that light scatters within it; the backlit behaviour described is Colourwise's reading of that, and no transmission figure is given because none was found in a reliable source. |
Applies to: Glass.
| Surface and finish | Light | Viewed | What changes | Why | How it goes wrong | What to do | Evidence |
|---|---|---|---|---|---|---|---|
| Clear window glass, glazed doors and glass-fronted cabinetsFinish: Transparent | Much brighter on one side than the other | Any angle | From the brighter side the glass behaves like a mirror; from the darker side, like a window. | Mirror-like (Fresnel) surface reflection. Glass always reflects a small share of the light that reaches it and transmits most of the rest. Which one you notice depends on the balance of light: the weak reflection of a bright room outshines what little arrives through the glass from a dark one.More: Glass colour: clear, low-iron, tinted and coated, Reflectance and transmittance measurement | A window that frames the garden by day shows only the room at night, and objects behind glass are hidden by reflections of a bright room. | Light what is behind the glass and dim what is in front of it; anti-reflective glass lowers the reflection itself. | Moderate evidence3 sources. The manufacturer's figure for the reflectance of ordinary clear float glass is about 8%; coated and tinted glasses reflect differently and are not covered. |
| Clear glass seen at a low angleFinish: Transparent | Any light | Along the surface (grazing) | Reflection takes over from transmission as the view approaches the plane of the glass, so the pane shows the sky or the room instead of what is behind it. | Mirror-like (Fresnel) surface reflection. The share of light reflected at a surface rises as the angle of incidence moves away from the perpendicular, steeply so close to grazing.More: Glass colour: clear, low-iron, tinted and coated, Gloss and colour | Shop windows and glass balustrades seen from along the street reflect the sky and hide what is behind them. | Arrange for glass to be seen more nearly face-on, or shade whatever would otherwise be reflected in it. | Moderate evidence1 source. A general property of reflection at a surface; no figure is given for a particular glass. |
| Clear float glass at its edge or in thick sectionsFinish: Transparent | Any light | Any angle | A green-blue tint that is hardly visible through the face of a pane becomes obvious at the edge and in thick glass. | Light absorbed on its way through the material. Iron impurities in ordinary glass absorb a little light at each millimetre. Through a pane's thickness the path is short and the tint slight; along an edge the path is long and the tint builds up.More: Glass colour: clear, low-iron, tinted and coated, Translucency and subsurface scattering | Glass shelves, table tops and splashbacks show a green edge, and white paint behind ordinary glass looks faintly green. | Specify low-iron glass where edges show or where a white or pale colour sits behind the glass. | Moderate evidence2 sources. Low-iron glass reduces the tint rather than removing it. |
These effects are real and familiar, but no source supporting the mechanism for the material was located, so they are named here rather than given a row.
Of the 29 rows, 14 rest on strong evidence, 14 on moderate and 1 on limited evidence. Strong means a standard, a peer-reviewed study, an institution or a manufacturer’s technical document states the mechanism for that kind of surface; moderate means it follows from a sourced general mechanism or rests on a reference work.