When a sequential optical design is being optimized or toleranced, we normally test system performance by using several analysis features that test the response of the system to an infinitesimal point on the object surface. Such features include ray fans, OPD plots, MTF, PSF and many more. The Geometric Bitmap Image Analysis feature lets you place a high resolution object scene on the object surface, and to trace rays from this scene correctly weighted by wavelength, field, apodization and source brightness. A schematic is shown below.

{Note that this image was produced by the non-sequential mode of ZEMAX: see the last page of this article for full details.}

The object scene is represented by a source bitmap, which can be a .BMP or .JPG file. Rays are traced using the defined object aperture and field towards the pupil of the optical system, and on through to the image plane (or other specified surface). In the detection surface we place a pixellated detector which receives the rays and builds up an image of the source bitmap as seen through the lens. This image includes the effects of all aberrations, vignetting at apertures and optionally of thin-film coatings and glass absorption too.

In additional to all the normal editor data, there are two pieces of information that ZEMAX must be given. The first is information on the size and resolution of the source scene. In the example we will use in this article, the source is a color LCD screen of specified dimensions and 640x480 (VGA) resolution. We will image this scene through three different optical systems, onto a detector which is also VGA resolution. Here is the source scene we will use:

 

and here is the settings dialog for the geometric Bitmap Image Analysis (GBIA) feature:

For full details of how to use the settings, see the User's Guide. Here is a brief description of the most important parts:

• Field Y Size defines the y-height of the source bitmap in whatever units the field is measured in. In our examples the field is object height in mm, and so the full width of the bitmap on the object plane is 13.8 mm. The x-width is then determined from the aspect ratio of the bitmap
• Parity allows us to account for systems that invert. This will re-invert the image so that we see it right-side-up, if we wish to.
• Input defines the source bitmap or .jpg scene that is to be used. This file must be located in the {zemaxroot}/imafiles folder
• Rays/Pixel determines how many rays should be traced from each pixel. This directly affects the signal/noise ratio, at the expense of calculation time. 1 ray/pixel is usually fine for setting the feature up correctly, 10 gives quick results, 100 gives very good images and 1000 or more gives photo-realistic images
• X-Pixels, Y-Pixels, X Pixel Size, Y pixel size allows you to define the size and resolution of the detector
• Show Source Bitmap lets you choose to render the input scene, and was used to produce the top graphic
• Output writes the resulting GBIA results to a .jpg or .BMP file. This is important because what you see via the Analysis window is viewed "through" your monitor's resolution, and so may be significantly downsampled or demonstrate Moiré. It also prevents you from losing data if you accidentally close the window before saving the results!