A camera setting (automatic or manual) that adjusts colours in a photo so that white objects actually look white under the light present at the time.
Different light sources have different colour temperatures; white balance compensates for this by shifting the colour cast of the whole image. An incorrect or automatic white-balance decision can shift every colour in a photo, including eye colour.
A colour looking different in two photos doesn't necessarily mean the real-world colour changed — it may simply reflect two different white-balance decisions made by the camera.
The same pair of eyes can photograph as more blue under one white-balance setting and more grey under another, without anything about the eyes themselves changing.
Explains why eye-colour photos (and colour photography generally) are not a reliable, definitive record of exact colour.
White balance is set as a pair of numbers: a colour temperature in kelvin along the blue-to-amber axis and a tint along the green-to-magenta axis. Underneath, it is a scaling of the camera's colour channels chosen so that something neutral in the scene comes out neutral in the image. The reliable way to find the scaling is to photograph a grey card in the same light as the subject. Automatic white balance has to guess, because a warm-coloured object under neutral light and a neutral object under warm light can produce identical image data.
| Light | Daylight balance left on | Balanced for the light |
|---|---|---|
| Daylight, CIE D65 | #767676 | #767676 |
| Incandescent tungsten, CIE A | #9c6c3a | #767676 |
| Cool-white fluorescent, CIE FL2 | #87725b | #767676 |
Under tungsten light a neutral grey card is recorded as #9c6c3a, an orange-brown, if the camera is still balanced for daylight. Balancing for the lamp returns it to #767676. The card reflects the same 18% at every wavelength throughout. The correction is a decision about which light to call white, applied to every pixel alike.
How this was calculated: A flat 18% reflectance is lit by each CIE illuminant and converted to sRGB twice: once as if the light were D65, once after Bradford adaptation from the light's own white to D65. Channels outside the sRGB range are clipped.
A room lit by a window on one side and warm lamps on the other is photographed for a property listing.
One global scaling can neutralise only one of the two lights, so either the window side goes blue or the lamp side goes orange, and the wall colour appears to change across the frame. Matching the light sources before shooting works better than correcting afterwards.
Online shoppers compare a product colour across sellers' photographs.
Each seller's camera made its own white-balance decision, so one item appears in several colours. A photograph is evidence of the colour only if it includes a known neutral.