Short answer
Print is prepared, measured and proofed for one agreed light: CIE illuminant D50, a daylight of about 5000 K, at a set level of illumination. Screens are built around the bluer D65. Colour management adapts between the two automatically, so nothing has to be converted by hand; what goes wrong in practice is that a print is checked under a warm lamp, a dim room or a bright window instead of the light it was made for. For everyday checks, indirect daylight is the nearest free substitute; for approvals, a D50 viewing booth is the reference.
A printed colour is not a fixed thing: the inks reflect whatever spectrum falls on them, and brightened paper glows more or less depending on the ultraviolet in the light. To make a proof approved in one building mean the same in another, the graphic arts fix the light. ISO 3664 specifies viewing conditions based on D50, with two levels for prints: a bright one of 2,000 lux for critical comparison of a proof with a press sheet, and a lower one of 500 lux for practical appraisal, closer to how the finished piece will be seen. It also asks for a neutral grey surround, because coloured walls change what the eye calls white. Measurement follows the same convention — the characterisation data behind every standard profile is CIELAB for D50 — so the numbers a printer measures and the colours a customer sees refer to one light.
A lamp sold as 5000 K has the right overall tint and nothing more. D50 is a defined distribution of power across the spectrum, and a viewing light is judged by how closely it follows that curve, in the visible and in the ultraviolet. Booth makers point out that most 5000 K lamps are not close enough to be used for judging colour. The ultraviolet part matters because of optical brighteners: the 2009 edition of the standard tightened it so that brightened papers look in the booth as they do in daylight, matching the M1 measurement condition. The 2025 edition, written for LED lighting, keeps those two conditions and adds a pair with the ultraviolet removed, for the many indoor settings where brighteners barely fluoresce; those correspond to measurement without ultraviolet.
CIELAB values are always relative to a white, and the table on this page computes the same screen colours against both. Neutrals do not change: white and grey are a* 0, b* 0 either way, because the adaptation maps one white onto the other. Saturated colours do: sRGB blue is about a* 79, b* −108 against D65 and a* 68, b* −112 against D50, a gap of twelve units between two descriptions of one colour. That is why a CIELAB value is only usable with its white stated, and why print values are D50 by convention. The ICC's profile connection space is D50 for the same reason, and every profile brings its device's white to D50 by chromatic adaptation. Skip the adaptation and the screen's white itself becomes a colour: measured against D50 it is about a* −2, b* −19, a distinct pale blue.
That last figure is what a D65 monitor looks like to an eye adapted to a D50 booth beside it, and why booth light looks yellow from the monitor's side.
For work that is not contractually colour-critical, the aim is to get near the reference cheaply. Use daylight from a window, out of direct sun, and stand where the sheet is evenly lit; avoid warm lamps, coloured walls and brightly coloured clothing near the print. Compare like with like: two prints side by side in the same light, never one remembered from another room. Dim the screen until a white page is about as bright as the paper beside it — at 500 lux a perfectly white sheet has a luminance of roughly 160 candelas per square metre, which is the range photo labs suggest for an editing screen. For a proof that will be signed off against a press sheet, none of this replaces a booth: that comparison is specified at the bright level, under a light that conforms to the standard.
| Colour | sRGB | L* / a* / b* relative to D65 | L* / a* / b* relative to D50 | Difference between the two sets of numbers |
|---|---|---|---|---|
| White | #ffffff | 100.0 / 0.0 / 0.0 | 100.0 / 0.0 / 0.0 | 0 |
| Mid grey | #808080 | 53.6 / 0.0 / 0.0 | 53.6 / 0.0 / 0.0 | 0 |
| Red | #ff0000 | 53.2 / 80.1 / 67.2 | 54.3 / 80.8 / 69.9 | 3 |
| Yellow | #ffff00 | 97.1 / -21.6 / 94.5 | 97.6 / -15.7 / 93.4 | 5.9 |
| Green | #00ff00 | 87.7 / -86.2 / 83.2 | 87.8 / -79.3 / 81.0 | 7.2 |
| Blue | #0000ff | 32.3 / 79.2 / -107.9 | 29.6 / 68.3 / -112.0 | 12 |
| Sky blue | #87ceeb | 79.2 / -14.8 / -21.3 | 78.9 / -17.5 / -21.8 | 2.8 |
| Tan | #c68642 | 61.2 / 18.0 / 45.6 | 61.6 / 20.6 / 46.0 | 2.6 |
| Condition | Used for | Illuminance | Ultraviolet | Matching measurement condition |
|---|---|---|---|---|
| P1 | Critical comparison of prints | 2,000 lux (± 500) | Included | M1 |
| P2 | Practical appraisal of prints | 500 lux (± 125) | Included | M1 |
| P3 (added 2025) | Critical comparison, no ultraviolet | 2,000 lux (± 500) | Excluded | M2 |
| P4 (added 2025) | Practical appraisal, no ultraviolet | 500 lux (± 125) | Excluded | M2 |
Why: The two lights have different spectra and ultraviolet content, so inks and paper brighteners respond differently in each.
Fix: Approve under D50 light, or at least compare proof and press sheet side by side in the same light.
Why: The monitor's D65 white is bluer and usually brighter, and the eye adapts towards it.
Fix: Dim the screen to the booth's level and look at one at a time, or calibrate the screen white to the paper for proofing work.
Why: The tool reports CIELAB relative to D65; print measurement is relative to D50.
Fix: Convert with a tool that states a D50 white, or exchange values together with their white point.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
StandardModerate evidence
As summarised by a viewing-booth manufacturer, ISO 3664 bases all print viewing conditions on CIE illuminant D50 and defines 2,000 lux (± 500) for critical comparison of prints and 500 lux (± 125) for practical appraisal, with a colour rendering index above 90 and limits on how far the light may depart from the D50 spectrum.
Caveat: Taken from a manufacturer's summaries of the 2009 and 2025 editions; the standard's own text was not read.
Source: New edition of ISO 3664:2025. What has changed?; Standardized colour matching to ISO 3664:2009; ISO 3664:2025 Graphic technology and photography — Viewing conditions
FactStrong evidence
The ICC profile connection space is D50: source and destination colour with any other white is transformed to D50 by chromatic adaptation, and colour is matched relative to the white the observer is adapted to, not as absolute luminance.
Source: Why is the media white point of a display profile always D50?
FactModerate evidence
The 2009 edition of ISO 3664 tightened the ultraviolet requirement for viewing lights so that optical brighteners in paper show as they do in daylight, and the 2025 edition added two conditions without ultraviolet that correspond to the M2 measurement condition, while the existing two correspond to M1.
Caveat: A manufacturer's account of the editions, not the standard's text.
Source: Standardized colour matching to ISO 3664:2009; New edition of ISO 3664:2025. What has changed?
FactModerate evidence
A 5000 K rating describes only a lamp's correlated colour temperature; D50 is a defined spectrum, and most 5000 K lamps do not follow it closely enough to be used for evaluating colour.
Caveat: Stated by a maker of D50 lamps; the distinction itself is standard colorimetry.
Source: The Difference Between 5000K and D50; Light sources and illuminants
Colourwise analysisStrong evidence
Computed from the sRGB definition, sRGB blue is about L* 32, a* 79, b* −108 relative to D65 and about L* 30, a* 68, b* −112 relative to D50 after Bradford adaptation, and an sRGB white described against D50 with no adaptation is about L* 100, a* −2, b* −19.
Based on: sRGB values converted to XYZ from the IEC 61966-2-1 primaries and D65 white, then to CIELAB against D65 directly and against D50 after linear Bradford adaptation, with Colourwise's colour library; see the table.
Caveat: The unadapted figure assumes an eye fully adapted to the D50 surround; in practice adaptation to a bright screen is partial.
Source: International Electrotechnical Commission (IEC) webstore and catalogue; Why is the media white point of a display profile always D50?
Reviewed 1 October 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.