Activity 4: High Dynamic range Imaging

I. Theory

While taking pictures using a digital camera outdoors under sunlight or in a place with a very bright artificial light source, have you noticed that after the shot is taken a very bright image is seen on the LCD screen? We sometimes refer to it as being saturated. This occurs mainly because of the limited dynamic range of the camera used. One has to choose the range of radiance values that are of interest and determine the exposure time suitably. Sunlit scenes and scenes with shiny materials and artificial light sources often have extreme differences in radiance values that are impossible to capture without either underexposing or saturating the film.

In this work, I will be outlining how to address this problem by using a technique called High Dynamic Range Imaging. The idea is simple. You take multiple pictures of the same scene under different exposure settings in order to recover the full dynamic range in such a scene. We then use the algorithm presented in the paper of Debevec and Malik, which fuses the multiple photographs into a single high dynamic range radiance map whose pixel values are proportional to the true radiance values in the scene.

This technique is very important as digitized photography is becoming increasingly popular and more widely used than its analog counterpart.

II. Experiment

We were fortunate that the Plasma Laboratory was very accommodating, allowing us to use their experimental set-up through which plasma sheets are created. The sample pictures were taken from an intensely bright plasma sheet under six (1/320 1/400 1/500 1/640 1/800 and 1/1000) different camera exposure values but with the same f-number. The f-number is inversely related to the size of the aperture, which can affect how irradiance is perceived by the sensor. We further observed that the camera is fixed (e.g. placed on a tripod) to avoid the need of aligning the images.

Figure 1. Plasma sheet images taken at 6 different exposure values: (a) 1/320 (b) 1/400 (c) 1/500 (d) 1/640 (e)1/800 and (f)1/1000

We picked 20 points in the image and plotted their ln (shutter speed) vs gray level value (Z's). The response function g(Z) of the camera was then solved using the steps outlined in Debevec and Malik. Therefore, given a set of pixel values obtained from twenty points in six images with different exposure times, this function returns the imaging system’s response function g as well as the log film irradiance values for the observed pixels.

Figure 2. System's image response g as a function of the observed pixels

Figure 3. Pixel values, Z, for different exposure times

Figure 4. Log film irradiance at diferent pixel values




I give myself a grade 0f 8.5 for this activity.

References:

[1] Maricor Soriano. Physics 305 Activity: High Dynamic Range Imaging 2009

[2] Paul Debevec and Jitendra Malik "Recovering High Dynamic Range Radiance Maps from photographs"

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