Atelier Bonryu(E)

infrared photography

 
 

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Laboratory: Infrared Photography

Remark #3 Sensitivity of a Non-converted Camera

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The sensitivity measurement: The purpose of this measurement is to determine  the value of (1/b) which is the relative sensitivity of the infrared light compared to the visible light.  The value of b is the overall effective attenuation ratio for the infrared light and it is, of course, more than unity.  For this purpose it is necessary to take an infrared and a visible light photographs of the same object at the almost same time by the same camera under the “correct exposure”.  But as it is difficult to define what is the “correct exposure” we fix up to be satisfied as the photograph is taken under a correct exposure if it looks sightly.  For these two photographs we write down the photographing parameters, i.e., the f-value (F), the shutter speed (T), and the ISO setting value (I).  If the incident light intensity and the transmission factor of the light due to the attenuation in the filters are L and A, respectively, the light intensity (P) at the surface of the sensor is derived as,

  

where c is a constant.  It should be noted that in an actual situation both the visible and the infrared lights have spectra of finite width wavelength and, therefore, to represent the light intensities by single values as P and L is only a rough approximation.  In a case of the conventional visible light photography visible lights attenuate slightly after passing through the color filters and the IR blocking filter fixed in front of the sensor.  We denote the transmission factor of the visible light due to this attenuation as
.  On the other hand, for the infrared light there is a large attenuation due to the IR blocking filter in front of the sensor and we denote the transmission factor of the infrared light due to this attenuation as
.  By the way, the intensities of the incident lights are, naturally, different for the visible and the infrared lights, even when the visible light and infrared photographs of the same object are taken at the same time.  To determine these values quantitatively some accurate measurement of the intensities of the incident visible and infrared lights separately is required.  However, as our purpose is only to obtain  a rough estimate of deterioration of the sensitivity for the infrared photography, we dare to assume that the intensity ratio of the visible light and the infrared light
is constant.  Moreover, though the light intensity at the sensor  P may have wavelength dependence for realization of appropriate exposure, we assume that the value P is the same for both the visible and the infrared lights.  Though by these assumptions we cannot make a quantitatively valid estimate of the sensitivity we can obtain a very rough estimate of the transmission factor A due to filters as shown in the following.


From the above equation the transmission factors due to the filters are given as

       

       

where
.  Though the value of the constant, k, is not known, it can be safely assumed that k is about the same for the visible light and the infrared photographs of the same object taken at the same time.  Consequently, we can define the effective attenuation ratio for the infrared light b as
       

The effective attenuation ratio for the infrared light b means the necessary multiplication factor of the exposure when one takes an infrared photograph.  In other words, when an infrared photograph is taken for the same object, the same ISO setting, and the same f-value as a visible photograph, b-times longer shutter speed is necessary.  Then, how much is the value of the effective attenuation b?

 

As described in the main text it is possible to guess whether by a non-converted camera an infrared photograph can be taken or not by observing the infrared light from a channel changer of a TV. Even if it is concluded by this test that the camera can be used for taking an infrared photograph, usability for the infrared photography depends on the camera.  It is rather difficult to determine quantitatively the usability of the camera for the infrared photography and we cannot answer this problem briefly.  Here, under some daring assumptions we derive a formula to calculate from experimental data the multiplication factor of the necessary exposure for the infrared photography by a non-converted camera.