These very handy discs are used to measure neutral color balance by reflecting the light falling onto your subject and into your camera lens to take a white balance reading.
What makes the CBL While Balance lens different is that it allows you to take an “incident” reading from the light source itself to calculate color balance, giving highly accurate results in all situations, even where the subject has a particular dominant color. The disc has two sides for specific purposes.
The White side is made using a specially formulated grooved board over a reflector lens, covered with a condenser, to give accurate color balance in mixed lighting situations, low light or on cloudy and overcast days.
The Grey reverse side is formulated from a secret combination of 13 different materials which have been tested to produce the most consistent color balance and is best suited to Studio Flash Photography and direct sunlight.