Where have all the CRTs gone?

Vision Scientists have used CRTs to display visual stimuli for over 50 years, but that's all set to change as the supply of new CRT-based displays has declined rapidly over the last five years.

Tom Robson, CEO Cambridge Research Systems, comments: "In 2005, CRT monitors like the SONY GDM F520 were at the pinnacle of their technology; after half a century of development it was the best it could possibly be at the lowest cost per unit. Only the largest companies could compete successfully in the market due to economies of scale. However, with no way to further cut costs or make technological improvements, it's simply not profitable to continue manufacturing them. Naturally, the same companies have invested in expensive new technologies to overcome the constraints of manufacturing CRTs and they now produce twice the size of LCD for half the price! You don't have to look very far to see the evidence. Just walk into any high-street department store that sells TVs and you'll immediately realise that all you can buy is an HD LCD TV: it doesn't matter which brand or screen size, the whole industry's gone LCD."

Some CRT monitors are still available

CRS acquired a small stock of 22" Mitsubishi Diamond Pro 2070SB colour CRT monitors. We are making these monitors available exclusively for Vision Scientists conducting colour science experiments using ViSaGe. However, if you want to display a solely luminance-defined stimulus, a small number of greyscale CRT monitors are still available from specialist manufacturers.

Click here to browse the full range of monitors from CRS

Using LCD monitors for vision reasearch

Although LCD computer monitors have been carefully designed to appear to work like a CRT, they generate their light output in a completely different way and their performance can differ significantly between between brands and models. This means that it is especially important to calibrate and fully characterise the specific display that you intend to use in your experiment. The Brainard, D. H., Pelli, D. G., & Robson, T. (2002) Display characterization section in: J. Hornak (Ed.) Encyclopedia of Imaging Science and Technology (pp. 172-188): Wiley notes that: "LCDs can be characterized similarly to CRT displays, but bear in mind the following points:

1. Angular dependence. The optical filtering properties of LCD panels can have a strong angular dependence, so it is important to consider the observer's viewing position when characterizing LCD displays. This is especially important if the observer will be off-axis or there will be multiple observers. Angular dependence may be the greatest obstacle to the use of LCDs for accurate rendering.
2. Temporal dependencies. The temporal response of LCD panels can be sluggish, resulting in more severe violations of the assumption of temporal independence than typically observed in CRT displays. For example, when measuring the gamma function, a different result will be obtained if one measures the output for each digital video value after a reasonable settling time than if one sandwiches one frame at the test level between two frames of full output. It is important to try to match the timing of the characterization procedure to the timing of the target display configuration.
3. Warm-up. The LCD panel achieves color by filtering a backlight normally provided by a cold-cathode fluorescent display. These behave similarly to CRTs when warming up, so be prepared to wait for 45 minutes after turning one on before expecting consistent characterization.
4. Channel constancy. Some LCD displays do not exhibit channel constancy. This does not appear to be inherent in LCD technology, however. Measurements of other LCD panels indicate reasonable channel constancy, at least as assessed by examining variation in the relative values of XYZ tristimulus coordinates.
5. Output timing. It can be important to know when the light is emitted from the display with respect to generating video signals. In a CRT-based display, the video signal modulates the electron beam directly, so it is easy to establish that the light from one frame is emitted in a 10 ms burst (assuming a 100 Hz frame rate) starting somewhere near the time of the frame synchronization pulse and the top of the screen, but this need not be so in an LCD system. To evaluate the output timing, arrange to display a single light frame followed by about ten dark frames. Connect one of the video signals (say the Green) to one channel of an oscilloscope and connect the other channel to a photodiode placed near the top of the screen. (Note that no electronics are needed to use the photodiode but the signal may be inverted and the voltage produced is logarithmically, rather than linearly, related to the incident light.) When observing a CRT, it will be possible to identify a short pulse of light about 1 ms or so wide located somewhere near the beginning of the video stream. If the detector is moved down the screen, the pulse of light will move correspondingly toward the end of the video frame. When observing an LCD panel, the signals look completely different. The light pulse is no longer a pulse but a frame-length block, and there may be a significant delay between the video stream and the arrival of the light. In fact, in some displays, the two may have no fixed relationship at all.
6. Resolution. Analog-input LCD panels (and projectors) contain interlace electronics that automatically resample the video signal and interpret it in a manner suitable for their own internal resolution and refresh rate. This is desirable for easy interface to different computers, but the resampling can introduce both spatial and temporal dependencies that make accurate imaging more difficult. If possible, LCD displays should be run at their native spatial and temporal resolution. Even then, it is not guaranteed that the electronics pass the video signal unaltered, and one should be alert for spatial and temporal dependencies. This consideration also applies to DLP displays.
7. Internal quantization. Analog-input LCDs may actually digitize the incoming video voltages with a resolution of only 6 or 8 bits before displaying it, so be prepared to observe quantization beyond that created by the graphics card.
8. Gamma. There is no inherent mechanism in an LCD panel to create the power-law nonlinearity of a CRT; therefore a power-type function does not work very well to describe the function relating digital video value to phosphor light intensity. One way to deal with this is to make measurements of the light output for every possible digital video value in and invert the function numerically."

In 2006, Kanchit Rongchai from the University of Cambridge, Department of Engineering worked as a Summer Intern for CRS. He conducted a thorough investigation of the characteristics of Liquid Crystal Displays (LCD) in the form of six experimental reports including warm-up behaviour, screen spatial non-uniformity, colour constancy and spectral analysis, gamma functions and the primary independence, spatial independence and temporal response. The aim of his project was to measure the characteristics of LCDs that are important for generating different types of visual stimuli and to compare the characteristics with CRTs. His report and conclusions are available as a PDF; the MATLAB scripts for ViSaGe that he used to characterise the LCD and CRT monitors are available on request (send email to enquiries@crsltd.com).

The next ten years and beyond

LCDs are set to be the dominant display technology for at least the next 10 years. Graham Prophet, Editor of EDN Europe explains why: "The real problem for any new technology in the sector is the internal combustion engine effect. While new technologies struggle to get to market, LCD becomes more entrenched as the high-volume standard, its manufacturing costs decline even further, and it becomes even more difficult for any new arrival to compete with." He continues: "You reach the point where the question is not, “Is this technology—SED, FED, OLED, or whatever—better than LCD?” but, “Is this technology better by a sufficient margin, given the—inevitably— higher price point at which it will enter the market, to displace LCD?” That’s not a display phenomenon, it’s true of any innovation in any market: but it’s particularly acute in this one because the key attributes are all— well… on display". To read Graham's full comments in the March 2008 edition of EDN, visit the EDN Europe website.

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