Grey cards and custom white balance for dental photography

Grey cards are reference cards that are used to calibrate camera software. It ensures that pictures produced by the camera have colours that match the object being photographed. Many factors can affect the colour of the object, the camera sensor just captures the light reflected through the lens so mainly, factors that affect the light source affect the colour. For non dental photography, ambient lighting e.g flourescent lights, weather conditions etc can introduce colours that affect the object. In dental photography, due to the strong flash needed for illumination, most of these external factors are  inconsequential. This can be seen if you expose a photograph with the flash off, you should expect to see a black image due to the small aperture, low ISO and high shutter speed. This means that room lights shouldn't affect the image significantly.

Potential flash factors that can affect object colour include:

-The plastic cover in front of the flash can yellow with time due to degradation of the plastic. White balance should be repeated every 6-12 months to account for this.

-Internal or external covers of softboxes aren't always pure white. They can become dirty or dusty and discolour with time.

-Flash bulb: Different flashes will have different tinges in their colour, if you change to a new flash then white balance needs to be recalibrated.

A grey card is by definition neutral coloured so if a photo is taken of the grey card with the normal lighting conditions, the grey card should appear perfectly neutral on the viewfinder. The method of white balance calibration is to expose an image of he grey card on a black background and check the RGB histogram. The peaks of each colour: Red, Green and Blue should line up with each other. If there is more blue for example, this indicates the flash has a blueish tinge to it. This image is then used for white balance calibration by going into the camera settings, selecting white balance calibration and selecting the image. Be sure to select customised white balance in the camera exposure settings. Then reexpose the same image and check the RGB histogram again. The peaks of the colours should now be lining up and the camera is calibrated. If you are shooting with RAW format images, the white balance can be corrected post exposure in a photography software. You do need a reference photo of a grey card to do this but the process is simple enough to do beforehand, set and forget.

When switching diffusers or to a polarised filter, you need multiple reference photographs for white balance as each method will produce different coloured flash light. Apparently Nikon allows you to set multiple customised white balances on preset modes to make the switching easier but I will have to experiment with this in the near future.