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!).
However, if a subject is not uniform (eg. very bright teeth in an otherwise dark oral cavity), the values could lead to photographs that don't represent the patient's lighting conditions. In the example above, the photos would be overexposed, as the exposure meter would have raised the exposure value for an overall mid-range brightness (expecting grey on grey).
relatively narrow variation in tooth colour puts very high expectations on any system defining their colour …)
You can smooth out such errors with a greyscale. The scale is placed as close as possible to the object or patient and the exposure is measured with the chart filling the field of view. At the same time, the camera should remain unfocussed. The QpCard shown here has three colours: Black, 18% Grey and White It enables light, shadow and middle tones to be set at the same time. If a real chart isn't available, a homemade "Greyscale" for checking white balance can be put together quite easily. You take a clean paper hankie and smoothly stretch it directly in front of the lens. Then, holding the camera in front of the light source, take an unfocussed (!) photograph. Lock/record the resulting colour and lighting settings. With every change of position, a new photo taken with a Greyscale chart will be required. The photos should always be accompanied by the image of the greyscale chart to act as a reference. In this way, the colours can be reproduced using the relevant software in the lab. In digital photography a greyscale chart can also be used for checking white balance. Clearly, every time there is a change in light settings, part of the grey chart needs to be photographed, in order to get a new reference point for the resulting photos.(see above)
Figure 3. Photograph showing colour before and after correction using the greyscale chart (centre field in figure 1).
A colour chart as in figure 4 works even better than a greyscale chart. Here there are two additional tones and six defined colours. The three vertical colours on either side of the chart works like 'traffic lights' and simplify the assessment process. (Author's note: The
Figure 4. Colour chart from Fotowand, Germany If two colours cannot be easily distinguished in the triplet on one side, then the triplet on the opposite field may be more distinct or vice versa. All colour fields should be equally visible. If not, a colour cast may result, or it may mean the chip receptor has a colour fault and is therefore unsuitable. It is important that all colour fields are equally visible and that the card is held directly in front of the camera. Take care to ensure that possible automatic settings do not immediately overwrite your settings. The best option is to test & use manual Mode. Despite taking every care when using exposure meters, colour checks and greyscales, the handling of colour in dental photography can only be an approximation. Even if you were to install the best calibration systems available to the graphics industry, alongside very expensive hardware, the human eye is still superior. The human eye can differentiate between ca. 13 million colours. (The Vita classic Shade Guide has, for example, 16 colours ;-) For those wanting to follow this up in greater depth, I recommend the „Farbfibel“ (Colour Bible) in figure 5. This pack contains everything you need to know about correct colour reproduction.
Figure 5. The „Farbfibel“ pack
You can find a few YouTube videos showing the use of Grey-cards here: http://www.youtube.com/watch?v=GfkM-LllSps Software: Photoshop, In English. http://www.youtube.com/watch?v=aCCYszvID CM&feature=related ( Software Lightroom in German http://www.youtube.com/watch?v=68VcZAU5N tw&feature=related This is a very good explanation on the use of Grey-cards: http://www.youtube.com/watch?v=sOxk8mhXG s0&feature=related (Software Lightroom, In English)
References: In English:. http://www.shop.stauferkreispanorama.de/shop/article_4/QPcard-101--15.html?shop_param=cid%3D7%26aid%3D4 %26 In German: http://www.shop.stauferkreispanorama.de/shop/article_4/QPcard-101--15.html?shop_param=cid%3D7%26aid%3D4 %26 Suppliers: Fotowand-technic, Germany http://fotowand.de/ Farbfibel, http://www.farbfibel.de/ If you have any further questions, we would be pleased to hear from you. TEETH’R’US
* “All cats are grey in the dark” Benjamin Franklin, Advice to a Young Man on the Choice of a Mistress (1745), full text at http://www.swarthmore.edu/SocSci/bdorsey1/4 1docs/51-fra.html Keywords: Exposure, greycard, shutter priority, Calibrating System, colour fault, photoshop, QpCard
About the author Ulrich Heker is the owner-manager of Ulrich Heker Dental Laboratory founded in 1996 with the strap line TEETH ‘R’ US. As a qualified master craftsman (German Master Dental Technician) since 1991, he has over 26 years’ experience both at the bench and in running a successful business. Ulrich lives in Mülheim on the river Ruhr and is an accomplished ‘western-style’ rider in his spare time. Ulrich is fluent in English and can easily be contacted at: Dentallabor Ulrich Heker D-45130 Essen Corneliastr. 17. T: +49 201 797 955, M: +44 7922 458299 Video: Skype : teeth.are.us Web: http://www.german-smile.info, http://www.dentallabor-heker.de ulrich@teethrus.de