Not all cats are grey in the dark* By: Teeth Are Us Germany Use of Calibrating Charts in the Surgery and Laboratory Patients' photographs can be a big help in communications between the surgery and the laboratory. They simplify the design process and offer an overview of the entire situation. Correctly exposed photographs are vital. Ulrich Heker describes how a greyscale chart and/or colour chart can simplify communication between the laboratory and the surgery. Essentially, 'correct' exposure is dependent on three factors:
The light that falls on the subject The lens, together with any filters The sensitivity of the film/chip.
Generally, the exposure isn't dependent upon the brightness or rather, reflectivity of the subject! Logically, the colour tones in the photograph should match those observed when the picture is taken. In order to achieve a brightness and colour reference, you can use a reference colour, the 18% Normal grey / Neutral grey that can be found on a greyscale chart. Greyscale charts are used in photography to calibrate exposure. They are usually made of cardboard or plastic, with one side coloured neutral grey and the other white. In Photography neutral grey is a grey tone that comprises all the colours of the spectrum equally. Such a grey tone is seen as 'colourless'.
Figure 1. Chart showing white, 18% grey and black
The grey side reflects approximately 18 percent and the white, around 90 percent of the light falling upon it. The special coating on both sides manages to maintain the same degree of reflectivity, even when exposed to a variety of different light sources
Figure 2. Various colour temperatures (Created for illustration purposes) 1.) 2.) 3.) 4.)
Flash > Neutral, Halogen Lamp > Yellow Daylight early morning > Blue Ambient light from fluorescent or energy saving bulbs > Green with banding created by alternating current
The illustrations above show why dentists and dental technicians generally use a Ring flash or twin flash bracket: They deliver colourneutral images. In dentistry and dental technology, photography is usually done using flash, which shows a tendency away from neutral towards blue. More recently LED lamps have become available, that do indeed offer the impression of strong light. However, they do not match the benefits of using a flash to achieve sufficient depth of field with a small aperture at fast shutter speeds. Nearly all exposure metres are calibrated such, that they automatically provide settings suited to the average brightness of a scene. The general aim is to obtain an average value for the brightness of the subject (Take care how you take measurements!).