CCD Test Methods The following data sheet is typical of that prepared by Apogee Instruments for each of its imaging cameras. Some of the measurements here are done by design, and some are performed on each camera in production. In this course, each test will be described and, where appropriate, tips will be given for performing these tests yourself. It is recommended that you familiarize yourself with the other subjects in CCD University before reviewing the subject of testing. These tests are important indicators of camera quality and they fulfill the commitment we have to our customers for substantiating the claims we make before the purchase. Gain The first section of data lists the gain of the system in electrons per count, also referred to as electrons/ADU. For a gain of 2.4, 10,000 counts of signal would equate to 24,000 photons of light. The gain figure is determined in section 6, and it will be discussed there. The standard deviation was obtained from the difference of two bias frames taken at -5 degrees C, calculated by software from a region of interest (ROI) near the center of the frame. The product of the gain and standard deviation is noise in electrons. A word of caution: Shutters can leak light. The camera should be well sealed and in a very dark room when this is done. Linearity Linearity is a measure of how consistently the CCD responds to light over its well depth. For example, if a 1-second exposure to a stable light source produces 1000 electrons of charge, 10 seconds should produce 10,000 electrons of charge. The deviation from this straight line is a measure of non-linearity. Dark count build-up over time has also been used by some, but we do not endorse this as a valid way to determine linearity. You can perform this test yourself to a reasonable degree. Try this procedure: Place the camera upright in a room with no windows and a stable light source. A fluorescent light will work. Position the light so that it casts indirect light on the ceiling above the camera. Cut out a piece of plain white paper and place it over the camera opening. Take exposures with the camera in even increments from zero until the camera saturates. Plot the counts on the Y axis and the time on the X axis. The counts can be converted to electrons by multiplying by the gain of the system. The line should be reasonably straight throughout the well depth of the sensor. A word of warning. If you are using a camera with a fast shutter that can go to 0.02 seconds, increments down to 0.1 seconds should produce results virtually free from shutter uncertainty. If you are using the ultra slow shutters only capable of 0.1 second exposures, then the increments of time should be larger (0.5 seconds). Try and keep the entire test very short (less than 5 seconds) and keep the camera as cold as possible to minimize dark count induced error. For Kodak KAF-0400 sensors at -5° C and 0.1-second increments for a total of 3.0 seconds to saturation, dark count is not a signficant factor. Under these conditions, very good linearity results can be obtained. Bias Level The bias level is shown on the Apogee Instruments data sheets for information only. It is the mean bias level taken from the statistics box on a raw bias frame discussed above during system noise calculations. Dark Count Dark count is a function of the CCD characteristics and the temperature of the CCD. The dark count will double with a rise of 5-6 degrees C. A simple way to determine dark count is to take a 60-second dark frame (a 60-second exposure with the shutter closed) at a temperature of -5° C. Determine the mean value of the pixels within a region of interest near the center of the frame. Next, take a bias frame and again determine the mean value. The dark count then becomes: (Dark - Bias)/60*system gain in electrons/second. A word of caution: Shutters can leak light. The camera should be well sealed and in a very dark room when this is done. Temperature Stability Temperature stability data given in the data sheets starts with a measure of the maximum temperature delta that can be achieved from an ambient temperature of +25° C. This maximum delta will often be greater than the difference between your ambient temperature and the operating temperature. The zero/scale numbers are those used by the software in interpreting the data coming from the temperature control subsystem. They are listed as a reference only. The Apogee Instruments temperature control system is independent of the computer and software once the system is enabled and desired temperature given. The computer can be reset, other programs can be run, and the camera-control software can be exited and re-entered without influencing camera temperature. Another feature of this temperature control system is the automatic ramp times to cold temperature and back to ambient. Once any desired temperature is programmed, the descent and ascent rate is controlled by the electronics. This limits the rate of change to the CCD, thus preventing premature failure due to excessive temperature shock. On the chart below, 15-20 minutes passes before the temperature ramps to the desired value of about -7 degrees C. The camera-control software reports current temperature by reading a status port on the temperature controller, but there is some noise in the reading of this temperature. For user information on the current state of the controller, this noise does not present a problem. For testing of real temperature stability, we directly monitor the temperature sensor telemetry. Unless you have electronics expertise and a precision voltmeter, we do not recommend you do this test yourself. Signal Variance The signal variance method of determining system gain in electrons per ADU is the most difficult of the tests discussed here, but can be repeated by anyone who follows the procedure outlined. This method is one where multiple exposures are taken with increasing light. Standard deviation and mean count data is collected for each image. The standard deviation numbers are each squared, then plotted with the net mean (mean - bias) numbers. The slope of the line represents the gain of the system. The test setup is very much like that discussed for the linearity test above. There are error sources in this simplified measurement and better methods exist for determining gain. This description is used for simplicity to better describe the concept. Try the following procedure: Set up the conditions as you did for the linearity test. The light will probably have to be brighter. Stack several sheets (8-10) of plain white paper over the camera. Take a bias frame. Record the mean and standard deviation from the cursor box positioned in the middle of the frame. Remove a sheet of paper. Take a 0.1 second exposure. Record the mean and standard deviation from the cursor box positioned in the middle of the frame. Repeat steps 4-5 until the paper is gone or the image saturates. Create 2 more columns next to the data you've recorded. In one column record the square of the standard deviation data you recorded earlier. In the other column, subtract the bias mean from each mean you recorded. Your data should now look something like: All that remains is to plot the net signal on the Y axis and the SD2 data on the X axis. Draw a straight line through the data to make the best possible fit. To determine the slope, pick 2 points along the line. The slope will be (Ypoint 1 - Ypoint 2)/(Xpoint 1 - Xpoint 2). For example in this case, the two points might be net signal numbers of 6000 and 0, and variance numbers of 2560 and 60. So (6000-0)/(2560-60) = 2.4 electrons per ADU (Analog to Digital Unit) or count.