The difference analysis between gray level and photographic density

The writer of gray lever and photographic density of Digital image: the gray of digital image is corresponding to photographic density of camera,the two have almost same function on ray detection---with the brightness change of different areas on images to change flux of light that reaches the eyes,make our eyes visible---but on quantity value change rule and flaw detecting ,the two exist difference.We know from learning film photographic theory,human eyes is respond to light intensity change is nonlinear,usually we call the smallest light intensity difference value needed by distinguished light difference into visibility threshold.Namely,when light intensity I improved,in certain amplitude it can be felt, it must up to certain value （I+ΔI）/I，human eyes can feel brightness has change，ΔI/I normally called contrast sensitivity.It seems to correspond to visual feeling ,the black degree formula is :D=lg（L0/L）, from this formula, human eyes correspond to black degree is linear relation.For example,if someone’s eye has 0.05 black degree change sensitivity，then this person just can identify  flaw image with black degree is 2.0;at the same time,identify flaw image with 3.0 black degree .But please mind you:the light difference are not same,the former is 112.2-100=12.2，the latter is 1122-1000=122, 10 times difference.The gray level of every pixel of digital image depends on binary value after electric signal analog/digital conversion .Due to not adopt logarithmic transformation analog/digital transformation,so the gray level of digital image and photographic density of file has different changeable rule,the former is equal difference rule,the latter is ratio of equality.It is not hard to make the gray level of digital image according to ratio of equality to change:we can use analog /digital transverter of logarithmic transformation to achieve this goal; or adopt easier way,after analog/digital conversion,using computer software to change gray level change from equal difference rule to ratio of equality.Then why nowadays DDA system doesn’t consider to adopt gray level mode with ratio of equality ?The answer is for digital image,gray level adopts equal difference or ratio of equality is not matter,the two modes has no big difference between theory research and practical application,because:1、the gray level of digital image is changeable,any gray level image can transform to comfortable level to observe by gray level,so it doesn’t need to make sure corresponding relation between gray level change and exposure;2、the contrast ratio of digital image can be improved,so it doesn’t need to build relationship between gray level change rule and contrast ratio to human eyes resolution ;3、signal to noise ratio of digital image has no relation with gray level，so it doesn’t need to build relation between gray level change rule and flaw identification and detection。Therefore for DDA system digital image ,some research content of film photograph,such as relation between blackness and signal to noise ratio 、relation between blackness and flaw identification、image blackness limiting value、 minimum visibility contrast are no longer applicable.

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