Stranica 23 [23]
OCR
(HT CHARISM A BI Z ole. TET the RAW image conversion software." An encoding gamma of 1/2.2 (such as those using SRGB and Adobe RGB 1998 colour) is commonly used (Figure 1-11(b)). In addition, Sshaped (tonal) curves may also be applied to the image if care is not taken to uncheck these options (see Chapter 3). These processes redistribute native camera tonal levels into ones closer to how the eye perceives them and add artistic enhancements, such as contrast and saturation, to the images. In order to produce comparable images, the linear data describing the tonal levels observed by the camera must be retrieved. (a) Recorded YV INPUT: Original Scene Luminance 0% 25% 50% 75% 100% INPUT: Actual Luminance Figure 1-11. (a) Comparison of the sensitivity of the human eye to light with that of a camera (b) Schematic of gamma function applied to an image by manufacturers. This depicts an image in the sRGB colour space (which encodes using a gamma of approx. 1/2.2).*° In the past, methods? to compensate for both camera-applied gamma and the spectral density of the radiation source have employed a set of Lambertian grey references such as the Spectralon references (Figure 1-12, for a description of Spectralon diffuse reflectance standards, see Chapter 2) inserted in the frame as shown in Figure 1-13. Spectralon references have certified reflectance properties with associated RGB values in the UV-VISIR spectral range under investigation.*? Capturing these in the image allows a calibration curve to be calculated to convert the original RGB values of the greyscale read on the image to the RGB values assigned to these internal standards. Figure 1-13 shows an example of a visible-reflected image which has been compensated for the distribution of the incoming radiation by applying such a calibration curve. The greyscale of the balanced image (right) shows the RGB values reported in the table in Figure 1-12. However, with this procedure the gamma function, which is a power function, is not truly removed but merely ‘compensated’ for. As a result, even simple mathematical operations undertaken with images corrected in this fashion will not provide the same results as with linear images, e.g. (A + B)" # A” + B". Even in the case of linear images, it should be mentioned that a gamma function is always finally applied to visualise them on a screen, which is intended to work with a gamma function to emulate what is observed by the eye. Version No. 1.0 15 Date : 14/10/2013
