Stranica 81 [81]
OCR
5) If site constraints allow, position the reference standards as recommended in section b. A scale may also be inserted in the image if required. 6) If working outside the studio or in the field, use the recommended procedure of “twin” images, described in section b. 7) Turn on the visible/IR light sources and ensure that both the subject and reference standards are evenly illuminated, checking with a light meter if preferred. 8) If using flashes, set these to their highest setting and check that these are synchronized with the camera. The room lights overhead may remain on. 9) If using continuous light sources such as tungsten lamps, turn the room lights off now. Camera capture settings 10) Camera settings will vary according to the equipment used but some general camera settings are recommended in section b. Exposure settings 11) The exact exposure settings will vary with the equipment used but some general exposure recommendations were made in section b. 12) Particular recommendations for acquiring IR-reflected images: (a) Aperture settings in the middle of your lens range (e.g. f8) are usually a good Starting point for maximum sharpness, but choose depth of field according to dimensionality of subject, as advised in section b; (b) The high irradiance of visible/IR radiation sources usually allows the use of smaller apertures (high f-stop number), especially if using flashes; (c) Shutter speed will vary according to whether continuous radiation sources or flashes are used but can generally be quite short (e.g. 1/8 sec for continuous sources and 1/200 for flashes). Adjust this as required for correct exposure. Obtain an image of the uniform reflective board (if site constraints allow) 13) Place the uniform reflective board so that it is evenly illuminated and fills the frame of the camera. As far as possible the board should be in the same plane as the subject so that the illumination on it is representative of the illumination on the object. 14) Focus on the board. Defocus slightly. Take the image. 15) Check the image exposure. The RGB values of the image on the image histogram (Figure 2-26) in playback mode are a good indicator. Note that these values pertain to the preview J PEG generated by the camera not the RAW data (see Chapter 3). Version No. 1.0 73 Date : 14/10/2013
