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What is high dynamic range (HDR) imaging?

Found in: Photography

HDR imaging combines several individual pictures, so that there is detail visible in both bright and dark areas. It increases the dynamic range of the picture.

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The dynamic range of a camera specifies the ratio between the brightest and darkest part of a specimen that it is still capable of capturing. Dynamic range should not be confused with the contrast of the picture. Cameras with a low dynamic range have a problem of accurately recording both bright and dark parts. The brighter parts of the specimen, such as fine structures that blend in with the background lit, will disappear completely. A reduction of the exposure time would make these bright structures visible, but now the darker parts of the specimen will be reproduced as completely black. It does not matter how we adjust the exposure time or light intensity, there will always be some loss of information either at the bright or at the dark end of the spectrum.

Digital cameras, in general, have a much lower dynamic range than analog film. It is, however, possible to digitally process the images to produce an overall final result which does not have any parts of the picture being over or underexposed. One has to take a sequence of several images, each one with a different exposure time. Some images will appear to be generally overexposed, with the darker parts of the specimen showing a proper brightness. Other images will generally appear too dark, with the brighter parts now showing more image details.

When taking pictures of a different brightness, The light intensity of the microscope should not be changed. this might result in a shift of color temperature. It is also not a good idea to change the setting of the condenser aperture diaphragm, as this also changes contrast and depth of field (the condenser diaphragm should generally not be used to control light intensity anyway). The computer program will then process these images to produce a final image with all parts being correctly exposed.

Before starting HDR imaging, one has to evaluate, whether the dynamic range of the specimen indeed surpasses the dynamic range capabilities of the camera. Most specimens are thin and quite thin and there are few parts which are dark enough to cause dynamic range problems. Whole mounts of Insects, however, are a different issue. The body is generally very dark and the fine hair and appendages are easily flooded by the light of the microscope. In these cases HDR imaging may be much more useful.

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