Lighting a Color Target

Its not rocket science; but the lighting of a color target requires a clear understandings of the functions, and the light sources, involved. This article will break down the various factors. The ideal way to light a target is with a single point light source at a great distance, at 45 degrees to the face of the target, with no other lighting. The rest is all details…

Why 45 degrees? Because the ideal response from the target patches is the “45/0” response, just like a spectrophotometer or colorimeter that illuminates the sample with light from 45 degrees, and measures the results from zero degrees, meaning perpendicular to the surface. There are other reasons this is ideal, including minimizing gloss reflections, offering a consistent balance between texture shadow and color, and avoiding complications that come with either straight-on lighting, such as an in-camera flash, or raking lighting, which accentuates shadows and unevenness.

Why a point light? Simply to make the light source as consistent and uniform as possible. Adding a reflector to a light that is very far away won’t cause problems, but simply moving it closer (if it does not offer enough light at that distance) and not adding the reflector would be a better choice.

Why a single light source? Its not impossible to light a target evenly with two lights, say from 45 degree angles to both sides, but its highly improbable that this method will actually provide even lighting. One theory of double lighting is that each light “falls off” a bit towards the farther edge of the target, and the second light, falling off in the opposite direction, will balance this out. The better way to avoid fall off, is to use a more distant source, such that the fall off is negligible. Secondary light sources are really about reducing shadows and harshness in images, and the image of a target is not intended to be pretty, its intended to be literal. Harsh is good, in this situation.

Other factors requiring mention here are the target and the camera, since it is the relationship of these two items with the light source creates the triangle that makes this all work. First, the target needs to be stably placed, perpendicular in both axes from the camera (having a tripod mount built into the target simplifies this). And the camera needs to be stably placed in relation to the target. Two tripods are the ideal solution, or a tripod for the camera and a light stand for the target. But with care, its possible to shoot an effective target image without either of these aids.

Now a bit on the “why” to go with the “how” above. Targets are shot to represent multiple things, or the workflow that the combination of these items creates. The light source is one factor, though a good “low metamerism” pigment target, once corrected for whitepoint and exposure, will offer very similar results under different light sources, assuming they are all fairly “good” (meaning smooth across the spectrum) lights. If you are shooting a particularly important set of images (say the entire clothing collection of an important commercial client) and want to be extra sure that you have the most accurate color results, a specific correction for this set might be justified, but in most cases, unless the light source is particularly difficult, a unique profile for the light source is unnecessary; all that is required is white balance and exposure data for the scene, which could be gathered from a color target, though other devices (particularly the Datacolor SpyderCube) are better suited for that job.

The next factor is the camera, by which we mean this particular camera unit; not the model line, but the physical camera in your hands. A generic camera color profile for a model line might be better than no correction at all, but it won’t address the unique issues of your particular camera, which is why custom profiling is important here.

If the camera’s lenses are interchangeable, and if you find the color is not quite the same amongst the lenses you own, then that is a final factor, one that may force you to create multiple profiles, for each lens, rather than a single one for the camera.

Now a bit on real world camera profiling. The photo below shows a perfectly acceptable target shot. There are no tripods involved, but the geometry is sufficiently accurate to do the job. The target is perpendicular, in both directions, to the camera, so there is little distortion of the target’s rectangle.

The light is single source, and comes from about a 45 degree angle to the target face. The light source is not a point, but its limited to a single patio door, with the hotel room blackout curtains closing off all but that one door’s glass. And the light source is a very spectrally smooth light source: the sky. While skylight is very blue (as high as 9300k), it is very even, and once white balanced and exposure corrected, this target capture will offer nearly identical patch values to one shot with a very warm but full spectrum light source, such as a tungsten lamp.

This particular target capture was taken because we were about to take a series of shots of artwork on this same coffee table, and needed reliable color in the resulting images, on a laptop which did not then have a color profile for the camera. This target capture did an excellent job of providing camera color correction for this series of images, and in conjunction with calibrating the laptop monitor, gave us sufficient confidence in our color to allow the entire project to be shot and processed on site.

Credits: C. David Tobie, Copyright 2012.   Website:   Return to Blog’s Main Page


One response to “Lighting a Color Target

  1. Pingback: Color Comparison: Canon 5D Mark ll and Mark lll | CDTobie's Photo Blog

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s