Using Uniformity (Light Falloff)

Measures light falloff (lens vignetting) and sensor nonuniformity

Light Falloff (Uniformity)  measures lens vignetting (dropoff in illumination at the edges of the image) and other image, illumination, and sensor nonuniformities. For example, it can measure evenness of flash illumination (using light bounced off a white wall) or the uniformity of flatbed scanners. In Imatest Master, Light Falloff also produces a pixel level histogram, analyses of hot and dead pixels and color shading, and a detailed image of fine nonuniformities (i.e., sensor noise). These features are described in Light Falloff instructions: Imatest Master .

Instructions

To prepare an image for Light Falloff,

• Set your camera or lens to manual focus and focus it at infinity (the worst case for light falloff).
• Photograph an evenly illuminated uniform subject.

One of the simplest ways to obtain a uniform subject is to photograph your computer monitor at a distance of 1-3 inches (2-8 cm) using the Imatest Screen Patterns module, shown on the right. Click on Screen Patterns on the right of the Imatest main window, then maximize the screen. You can adjust the Hue, Saturation, and Lightness (H, S, and L, which default to 0,0,1) if required. (Thanks to Jonathan Sachs for this suggestion.) This method works best with flat screen LCDs with wide viewing angles. For extreme wide angle lenses or laptop screens with a limited viewing angle, an additional diffuser should be used. We recommend using opal diffusing glass mounted close to the lens. Opal glass is available in the US from Edmund Optics. (Thanks to Bart van der Wolf for the suggestion.) If a diffuser is used, a lightbox may be substituted for the monitor. (Lightboxes are typically brighter but less uniform.)

Screen Pattern module for Light Falloff.

This technique is less effective with CRTs. L may need to be reduced to keep exposure time longer than 0.1 seconds so the raster scan doesn't cause uneven exposure. The Canon EOS-40D exposure metering seemed to fail as a result of the raster scan. (We prefer the monitor, but this approach may be useful in come cases.) Photograph an evenly-lit smooth sheet of matte paper (white or gray). Get close to the subject, but not so close that you shade it. Outdoors illumination (shade) sometimes results in the most even illumination. Getting uniform illumination can be a challenge with ultrawide lenses, such as the Canon10-22 mm digital zoom lens set to 10mm (equivalent to 16 mm in the 35mm format); it's almost impossible to avoid shading part of the card. The computer monitor (above) is strongly preferred in such cases.

• The subject does not need to be in focus (you don't even need a lens to measure sensor uniformity); the goal is to measure lens light falloff and/or sensor uniformity, not features of the subject.
• For typical lens vignetting measurements, set exposure compensation to overexpose by about one f-stop. (You may, however, use any exposure you choose.)
• Save the image as a RAW file or maximum quality JPEG.

To obtain truly even illumination for precision scientific measurements you'll need an integrating sphere from a supplier such as SphereOptics, Labsphere, or Electro-Optical Industries (EOI). The SphereOptics system on the right is about $8,000, including variable attenuator, power supplies, operating software, and calibration. It is available in sizes from 4 to 76 inches. The Labsphere USS-1200 or USS-1200V and the EOI ISV 400, all 12 inches in diameter with 4 inch exit ports, are ideal for SLRs. Smaller models will work with compact digital cameras. Integrating spheres aren't cheap. Less expensive alternatives include Kyoritsu calibrated light sources (especially the pattern light boxes), available in the North America from C.R.I.S. Tsubosaka (Japan) also has some interesting products. There are no obvious US or European distributors.

To run Light Falloff,

• Start Imatest and click on Light Falloff.
• Very large files (height x width x colors over 40 MB) may cause memory overflow problems. Files over 40 or 80 MB can be automatically reduced 1/2x linearly (using 1/4 the memory). Click Settings, Options and Settings I... (in the Imatest main window) and make the appropriate setting in LARGE FILES (Light Falloff, Distortion).
• Open the image file.
  
  

  Batch mode  This module can operate in batch mode in Imatest Master, i.e., it can read multiple input files. All you need to do is select several files using the standard Windows techniques of shift-click or control-click. There are three requirements for the files. They should (1) be in the same folder, (2) have the same pixel size, and (3) be framed identically. The input dialogs (cropping (if applicable), settings, and save) are the same for the first run as for standard non-batch runs. Additional runs use the same settings as the first run. Since no user input is required they can run extremely fast. If the order of the files in a batch runs is different from the selection order, click Settings, Options and Settings... (in the Imatest main window) and change the setting in Batch run order. One caution: Imatest can slow dramatically on most computers when more than about twenty figures are open. For this reason we recommend checking the Close figures after save checkbox, and saving the results. This allows a large number of image files to be run in batch mode without danger of bogging down the computer.

  
  
• Three cropping (ROI selection) options are available by clicking Settings, Options and Settings... in the Imatest main window. These include
  
  

  Don't ask to crop. (Use settings in LF input dialog box.)

  Select crop by dragging cursor. Ask to repeat crop for same sized image.

  Select crop by dragging cursor. Do not ask to repeat crop.

  
  Although the first option (Don't ask to crop...) was the default prior to Imatest 2.3.12, the second option (Select crop by dragging cursor. Ask to repeat crop for same sized image), which is similar to the ROI selection in SFR, may be preferred.
• A dialog box appears. The following settings appear in all Imatest versions. Imatest Master settings are shown below.

• Title  Defaults to the file name. You can change it if you wish.

Display Light Falloff contour plots (Plot area; on by default). Turns the main light falloff contour plots off or on. Normally on (checked), but may be turned off by Imatest Master users interested in other results, such as hot/dead pixels and color shading. Four display options are available. Normalized refers to the first (level) plot only.   Contours superposed on image (not normalized) Contours superposed on image (normalized to 1) Pseudocolor with colorbar (not normalized) Pseudocolor with colorbar (normalized to 1)

Input dialog box in Imatest Studio

• Gamma  (Settings area). The default value of gamma, 0.5, is typical for digital cameras. Gamma affects the second figure (the light falloff measured in f-stops); it has no effect on the first figure. Gamma can be measured by Stepchart using any one of several widely-available step charts. (Reflection charts are easiest to use but transmission charts can also be used to measure dynamic range.) Some issues in calculating gamma are discussed below the second figure.
• Corner and side regions  (default 32x32 pixels) allows you to select the areas at the corners and sides of the images to be analyzed. Choices include 10x10 pixels, 32x32 pixels, 1% (min. 10x10), 2% (min. 10x10), 5%, and 10%.
• Crop pixels near borders (L, R, T, B)  (Settings area). If checked, the image is cropped by the number of pixels indicated near the left, right, top, and bottom borders.

The following options are available in Imatest Master only. They are discussed in detail in Light Falloff instructions: Imatest Master .

• Channel  (Settings area) You can choose between Red, Blue, Green, and Y (luminance) channel
• Hot and dead pixels  (Settings area) By checking the appropriate boxes you can display hot pixels (red x) and/or dead pixels (blue •). Hot pixels are stuck at or near the sensor's maximum value (255 in 8-bit files) and dead pixels are stuck at or near 0.

Because signal processing— especially JPEG compression— can cause these values to shift, you can use the sliders to set the detection threshold between 6-255 for hot pixels and 0-249 pixels for dead pixels. (The extreme values are for measurements made on white or black fields.) Clicking on < or > at the ends of the sliders adjusts the threshold by 1. The default values are 252 and 4, respectively. Settings are saved between runs. JPEG files must be saved at the highest quality level for this feature to work; isolated hot and dead pixels tend to be smudged at lower quality levels. Details are described in Light Falloff: Imatest Master .

Input dialog box in Imatest Master

• (Plot area) Color shading  displays color nonuniformity. Several options  are available. Display Histogram  displays histograms of R, G, and B channels. Display color uniformity profiles  displays R, G, and B values (or ratios— several options available) along the diagonals and horizontal and vertical center lines. Display fine detail  displays a detailed figure of noise and sensor uniformity. The calculation can be slow and uses lots of memory. Details in Light Falloff: Imatest Master .

Results

The first figure: luminance contour plot

shows normalized pixel level contours for the image file luminance channel, where luminance is defined as Y = 0.30*R + 0.59*G + 0.11*B.. A maximum value of 1 corresponds to pixel level = 255 for image files with a bit depth of 8 or 65535 for a bit depth of 16. Some illumination nonuniformity is evident in the plot: the top is brighter than the bottom. The image is smoothed (lowpass filtered) before the contours are plotted.

The text displays the maximum normalized pixel level for the luminance channel, the worst and mean corner values (in normalized pixel levels and as a percentage of maximum), and the side values. Selected EXIF data is shown on the right. Two hot and two dead pixels (which were simulated) were detected with thresholds of 246 and 10 (pixels), respectively. The crop (Left, Right, Top, Bottom) is shown just below. Details below.

The setting for correcting light falloff in the Picture Window Pro Light falloff transformation is also given. The PW pro Light Falloff dialog box is shown on the right. Film Size (mm) remains at 36 (the PW Pro default value: the width of a 35mm frame). Picture Window Pro is the powerful and affordable photographic image editor that I use for my own work. The Lens Focal Length is rarely the exact focal length of the lens. Light falloff depends on the lens aperture (f-stop) as well as a number of lens design parameters. Lenses designed designed for digital cameras, where the rays emerging from the rear of the lens remain nearly normal (perpendicular) to the sensor surface, tend to have reduced light falloff.

For aesthetic purposes I generally recommend undercorrecting the image, i.e., using a larger Lens Focal Length. This makes the edges somewhat darker, which is usually pleasing. Ansel Adams routinely burned (darkened) the edges of his prints. Part of the reason was that he had to compensate for light falloff from his enlarger (when he wasn't contact printing).

"My experience indicates that practically every print requires some burning of the edges, especially prints that are to be mounted on a white card, as the flare from the card tends to weaken visually the tonality of the adjacent areas. Edge burning must never be overdone..."

Ansel Adams, "The Print," p. 66. 1966 edition.

The second figure: f-stop contour plot

shows image file luminance contours, measured in f-stops, normalized to a maximum value of 0. A pseudocolor display with color bar has been selected. The colors in the color bar are fixed: colors always vary from white at 0 f-stops to black at -4 f-stops and darker. For this plot to be accurate, a correct estimate of gamma (the camera's intrinsic contrast) is required. Gamma is measured by Stepchart, using any one of several widely-available step charts, or by Colorcheck.

Gamma can be tricky to measure for several reasons. (1) Many cameras have complex response curves, for example, "S"-curves superposed atop gamma curves. This means that gamma can vary with brightness. (2) Some cameras employ adaptive signal processing in their RAW conversion algorithms. This increases contrast (i.e., gamma) for low contrast subjects and decreases it for contrasty subjects. This improves image quality for a wide range of scenes, but makes measurements difficult, especially since Light Falloff targets have the lowest possible contrast.

Additional figures are illustrated in Light Falloff: Imatest Master .

.CSV and XML output files

The .CSV output file contains additional statistics. Most have obvious meanings.

• Image pixels contains the width, height, and total size in pixels.
• Hot and Dead pixels show the total count and the fraction (divided to total pixels)
• The x and y coordinates of the hot and dead pixels are listed. The maximum is 100. Coordinates are in pixels from the top-left.

Contact Imatest if you need additional .CSV output.

The optional XML output file contains results similar to the .CSV file. Its contents are largely self-explanatory. It is stored in [root name].xml. XML output will be used for extensions to Imatest, such as databases, to be written by Imatest and third parties. Contact us if you have questions or suggestions.

Saving results

At the end of the run, a dialog box for saving results appears. It allows you to select figures to save and choose where to save them. The default is subdirectory Results in the data file directory. You can change to another existing directory, but new results directories must be created outside of Imatest— using a utility such as Windows Explorer. (This is a limitation of this version of Matlab.) The selections are saved between runs. You can examine the output figures before you check or uncheck the boxes. Figures, CSV, and XML data are saved in files whose names consist of a root file name with a suffix for plot type and channel (R, G, B, or Y) and extension. Example: IMG_9875_ISO1600_RGB_f-stop_ctrG.png. The root file name defaults to the image file name, but can be changed using the Results root file name box. Be sure to press enter. Checking Close figures after save is recommended for preventing a buildup of figures (which slows down most systems) in batch runs. After you click on Yes or No, the Imatest main window reappears.

Links

Vignetting by Paul van Walree, who has excellent descriptions of several of the lens (Seidel) aberrations and other causes of optical degradation.