If you have a display that needs to be measured, you are probably curious if a conoscope would be the right tool for the job. Conoscopes have been used for characterization of liquid crystal displays for decades. They are typically used to take measurements of viewing angle and contrast, and they do so with speed and simplicity. But no instrument is perfect, so conoscopes also have some constraints. We believe you will find that the advantages to using a conoscope to test a display far outweigh the shortcomings.
Conoscopes are most frequently used to measure the contrast and viewing angle of displays. With a conoscope, it is possible to capture in a single frame the light coming from almost the entire range of angles emitted by a display. For example, an 80° conoscope can measure light that is emitted from the normal (perpendicular) to the display all the way down to 10° from the display plane, and over the entire 360 degrees in azimuth. This light is captured into a circular image on the image sensor, from which it can be read out with resolution of approximately 0.1°, depending on the resolution of the image sensor. To make a viewing angle measurement, the display is set to white and the brightness falloff versus angle is measured. If this measurement is repeated with black pixels and the ratio of white to black is calculated, a contrast map is generated.
Speed and simplicity are the primary reasons for using conoscopes to measure display performance. The most common alternative is to scan a single pixel detector through the hemisphere into which the display emits light. To fully characterize a display with a single pixel detector takes millions of individual measurements for each area of interest on the display. This is a time-consuming procedure, so it is rarely used. Capturing all of this data in an image with several megapixels in a fraction of a second makes it possible to characterize many areas of the display in a few minutes.
The two major constraints on a conoscope for use in measuring displays are the requirement for a pupil in front of the lens and etendue. The first of these means that the conoscopic lens must be designed so that an image of the stop (a circular aperture within the lens that limits the amount of light collected at each angle) is formed on the surface of the display. This constraint is primarily on the design of the conoscopic lens, but it does mean that a conoscopic lens designed for measuring small light sources will not work well for measuring displays. Stray light will seriously affect any measurements. The second major constraint, etendue, is a limitation on the design of conoscopes that is often frustrating for customers. It is based on conservation of energy, so there is no way to get around it. The end result is that once you have chosen the sample area and collection angle of a conoscope, you have also constrained the smallest image sensor you can use. Or, if you choose the image sensor and collection angle, you have limited the size of the sample area. The equation relating these three properties is:
Φ(sample) * sin(angle) = Φ(sensor) / 2*f/#
Φ(sample) = the diameter of the sample,
angle = the collection angle of the conoscope,
Φ(sensor) = the size of the image circle, and
f/# is the f/number of the conoscope.
It is increasingly difficult to design a conoscope when the f/# drops below 2, so it is best to treat the f/# as a fixed quantity with a value of 2. The sine of 70° is 0.94, so sin(angle) may be approximated by 1 for any angle greater than that. This means that a good rule of thumb is that the shortest dimension of the image sensor should be about 4X the sample size. For example, a 2 mm diameter sample will require a 1” image sensor.
Conoscopes are typically designed with one function in mind: mapping the light from the display emitted into various angles into a circular image. If this is the only thing the conoscope does, the user gets no optical feedback when the distance between the conoscope and the display is properly set. To overcome this problem, auxiliary optics can be built into the lens that will also create an image of the sample. Another option is to design the conoscope to enable polarization measurements. Unfortunately, this adds a lot of complication to the design process, so this is a very expensive option.