White Coat Balance

June 7, 2010 § Leave a comment

Getting the correct white balance is probably more important in medical photography than any other form of photography.White balance is the process of adjusting the camera or an image to remove color casts made by light sources. All light sources, from the sun to a bedside lamp, have color casts ranging from blue to yellow. Sunlight is cold (blue) while the tungsten bulb in a bedside lamp emits warm (yellow) light.

The human eye is very good at adjusting for these color casts, and we perceive a piece of white paper as being white both out in the sun and under a light bulb. Digital cameras , however are not as good as our eyes. So we have to do a white balance of the photo either in camera or in photo editing software after the photo is taken.

In white balancing we essentially tell the camera/software what should be rendered as white or color neutral and the camera/software will adjust the colors of the entire image accordingly. The white balance can be used as a creative tool, deliberately setting it a bit too yellow to create a warm mood for example. But in medical photography we want to reproduce reality as correctly as possible. Changes in color of the skin or wounds are essential symptoms in many diseases, so correct white balance is crucial to scientific photographic documentation.

(For a more in-depth explanation of white balance and color temperature go here.)

When it comes to medical photography there’s only one way of white balancing that will ensure correct reproduction of colors: Shooting RAW files and using a neutral reference – a gray card (see top photo).

This is a photo of my hand shot in our studio with two studio flashes. The camera white balance is set to “flash”, so you would think would do the trick. The camera know it’s shooting in flash light and removes a bit of blue to keep the colors right. And the photo looks correct, doesn’t it?

It’s not. No two light sources have the same color, no two flashes are the same. So to get this correct we include a neutral reference in the photo, which we’ll use to get the correct white balance in the photo editing software.

Here I’ve hung a SpyderCube on my hand. This is a cube with its sides painted neutral gray, neutral white and neutral black (as gray cards can also be used to determine exposure, but that is another post). I take a shot of this in addition to the one of my hand without the cube. I’ll use the one with the cube to find the correct white balance.

I import the photos into my photo editing software (Adobe Lightroom), and the use the white balance eye-dropper tool to select the neutral target in the photo. I pick a portion of gray. The photo is adjusted and I can apply the same white balance settings to the photo without the cube.

There’s a huge difference even though the camera settings were “correct”:

And if the colors can be this off in the studio, just imagine how important this is when your shooting out in the wards with mixed light sources (fluorescent, flash, sunlight through window etc.). The colors can be only slightly off to reproduce skin tone variations erroneously.

Now for the title of this post. Working in health care does provide a white balance advantage when it comes to shooting interiors, groups and portraits – the white coat and scrubs.

This is a part of a photo shot in fluorescent light with the white balance set to “flash”, which results in a yellow image with a hint of green. Picking the white coat as the neutral target will remove the color cast and the coat will be rendered white.

Now, anyone who’s worked in a hospital know that the hospital clothes are not always perfectly white – but that does not matter. What matters is that they should be perceived as white and therefore used as a neutral target.

This process is actually not something new. Centuries ago women started to finish the washing of white clothes by dying them slightly blue to remove the yellowish hue that they got after being used some time. White balance.

Tagged: gray card, lightroom, medical photography, spydercube, white balance

The Sterile Eye