Cathode-ray tube
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A cathode-ray tube (CRT) is a vacuum tube that produces an image by firing a beam of electrons from a cathode at one end of the tube onto a phosphor-coated screen at the other, where the phosphor emits light at the point of impact. The German physicist Karl Ferdinand Braun invented the tube in 1897, and his cathode ray oscilloscope was the forerunner of the television tube.[1][2] CRT monitors were long the dominant display devices in fields such as vision science, until liquid crystal displays largely replaced them.[3]
Miniature CRTs were the main image source in early head-mounted displays. Ivan Sutherland's 1968 head-mounted three-dimensional display used two of them, and early research and military helmet-mounted displays mostly used small monochrome tubes before active-matrix liquid crystal displays and later microdisplay technologies replaced them.[4][5] Projectors with CRT projection tubes also drove the original CAVE in the early 1990s.[6] Because a CRT emits each frame as a short pulse of light rather than holding it steady, it is a common reference point in discussions of display persistence and motion blur.[3]
How it works
In its simplest form a CRT is an evacuated tube with an electron source, the cathode, at one end and a phosphor screen at the other, with internal or external means to accelerate and deflect the electrons. The assembly that produces the beam is called the electron gun.[1] In the CRTs described in the U.S. Army's 2009 reference on helmet-mounted displays, the beam is steered to the desired point on the screen by a magnetic field from a deflection yoke, which has separate coils for horizontal and vertical deflection; the deflection is set by the current in the coils.[1] Sutherland's miniature tubes instead used electrostatic deflection and focusing, driven by deflection plates that needed signals of only about 300 volts.[4]
Phosphor grains struck by the beam emit light. Color monitors use three phosphors, each emitting a different wavelength.[7] A CRT can draw an image in two ways. A raster display sweeps the beam in rows from the top left to the bottom right of the screen, so the whole image is never updated at the same moment.[7] A line-drawing display instead moves the beam directly between the end points of each line segment; Sutherland's headset used this method.[4]
The Army reference ranks the CRT near the top among display technologies for luminance, resolution and flexibility of addressing, and at the bottom for size (mainly depth), weight, high anode voltage, power requirements and heat generation.[1] A CRT also lacks full pixel independence: Ghodrati, Morris and Price note that the relatively poor spatial independence of adjacent pixels on CRT monitors introduces artifacts in stimuli with high spatial frequency.[7]
History
Braun described his tube in the paper "Ueber ein Verfahren zur Demonstration und zum Studium des zeitlichen Verlaufes variabler Ströme", published in Annalen der Physik in 1897.[8] The IEEE History Center's biography of Braun calls the cathode ray oscilloscope his best-known invention and describes it as the forerunner of the television tube and radar tubes. In Braun's tube, the end of the tube was coated with a fluorescent material that glowed where the electron stream struck it, and a magnet outside the tube moved the beam back and forth. Braun shared the 1909 Nobel Prize in Physics with Guglielmo Marconi for their work on radio.[2]
Vision researchers long treated CRT monitors as the gold standard for presenting visual stimuli.[7] Competition from plasma screens and liquid crystal displays led most manufacturers to reduce or stop CRT production during the mid-2000s, and by 2012 Elze and Tanner described CRT devices as largely replaced by LCDs in vision science.[7][3] The Army's helmet-mounted display reference likewise records that flat panel displays replaced CRTs as the display technology of choice over the two decades before its 2009 publication.[1]
Use in head-mounted displays
Early designs
Morton Heilig's US patent 2,955,156, "Stereoscopic-television apparatus for individual use", was filed on 24 May 1957 and granted on 4 October 1960. It describes a head-worn casing holding a pair of optical units, a pair of television tube units ("preferably a colored tube"), earphones and air nozzles, with a lens system meant to fill an arc of more than 140 degrees horizontally and vertically.[9]
Ivan Sutherland's head-mounted 3D display, described in a paper at the AFIPS Fall Joint Computer Conference in December 1968, attached special spectacles containing two miniature cathode ray tubes to the user's head. Each tube formed a picture about half an inch square, and the optics magnified it into a virtual image about eighteen inches in front of each eye, giving a 40 degree field of view. Half-silvered mirrors in the prisms let the wearer see the CRT images and the room at the same time, an arrangement now known as an optical see-through display. Sutherland wrote that the tubes had a nominal spot size of six tenths of a mil, which gave the virtual image a resolution roughly equal to that of standard large-tube displays.[4] Each tube sat in a grounded metal can to protect the user from shorts in the high-voltage system. A commercial analog line-drawing display supplied the deflection signals; it drew long lines in a nominal 36 microseconds and short lines in as little as three.[4]
Military helmet-mounted displays
In helmet-mounted displays for aviation, early image sources were miniature CRTs or image intensification tubes.[1] The Army reference states that military applications were a primary driver for miniature CRTs, which were developed in half-inch and one-inch diameters. Citing a 1997 comparison, it reports that the one-inch tube offered the best raster resolution and luminance, with a representative resolution of the order of 800 x 600, but that even a miniature CRT has a weight, volume and power footprint that makes it a difficult choice for a head-worn display.[1]
According to a 2015 Road to VR profile of Thomas Furness, the Visually Coupled Airborne Systems Simulator (VCASS) that he developed for the US Air Force was demonstrated in 1982 and used one-inch diameter CRTs that accepted images with two thousand scan lines.[10]
The Army reference documents several fielded and planned systems:[1]
| System | Platform | CRT image source | Notes |
|---|---|---|---|
| Integrated Helmet and Display Sighting System (IHADSS) | AH-64 Apache helicopter | One 1-inch diameter CRT with P43 phosphor | Monocular (right eye); imagery from a nose-mounted forward-looking infrared sensor is relayed through optics and reflected off a beamsplitter into the pilot's eye; filters suppress the phosphor's unwanted red and blue side lobes |
| Joint Helmet Mounted Cueing System (JHMCS) | F-15, F-16 and F/A-18 | One half-inch CRT | Visor projection, monocular (right eye only), daytime stroke symbology, electromagnetic tracker, 20 degree field of view |
| Helmet Integrated Display Sighting System (HIDSS) | RAH-66 Comanche helicopter (program cancelled in February 2004) | Two 1-inch CRTs in the first design | Designed by Kaiser Electronics in the 1990s; the second CRT pushed head-supported weight beyond the Army's safety limits, and a follow-up design used miniature LCDs instead |
| F-35 Helmet Mounted Display System (HMDS) | F-35 | None | Replaced the JHMCS CRT with two 0.7-inch SXGA active-matrix LCDs, with a 40 by 30 degree field of view |
The same reference notes that in 1991 the Defense Advanced Research Projects Agency (DARPA) set up a head-mounted display initiative within its High Definition Systems Program to develop display technologies that would overcome the limitations of CRTs.[1]
Research and commercial VR headsets
In their 2005 review of head-mounted display systems, Jannick Rolland and Hong Hua write that early HMDs primarily used miniature monochrome CRTs and that a few designs implemented color field-sequential CRTs. VGA-resolution active-matrix LCDs then became the source of choice, followed by higher resolution LCDs, ferroelectric liquid crystal on silicon, OLED and other microdisplays.[5]
At NASA Ames Research Center, I. E. McDowall, Mark Bolas, S. Pieper, S. S. Fisher and J. Humphries described the Counterbalanced CRT-based Stereoscopic Viewer (CCSV) in 1990. The viewer combined a dual-CRT stereoscopic display and wide-angle optics with a counterbalanced linkage whose joint angles a dedicated microprocessor monitored. The authors wrote that it suited users who often moved between desk work and virtual environment viewing, who needed high resolution views, or who shared the viewer in a group.[11]
Virtual Research Systems used both CRTs and LCDs in its commercial headsets. Its 1998 company profile says the EyeGen3, introduced in spring 1993, used miniature CRTs to provide high resolution at a mid-range price, and that the 1994 VR4 moved to liquid crystal displays. The company's high-end FS5, unveiled at SIGGRAPH 95, used CRTs with 800-line horizontal resolution; in 1997 the V8 replaced it.[12] The FS5 product page lists dual one-inch black and white CRTs with color shutters, field sequential input, a 55 degree diagonal field of view at 100% overlap or 76 degrees at 50% overlap, an interpupillary distance range of 52 to 74 mm and a weight of 38 oz (1.07 kg).[13] Color shutters in front of a monochrome tube are one implementation of field sequential color.
Projection systems
The CAVE built at the University of Illinois at Chicago's Electronic Visualization Laboratory used rear-projection screens for three walls and a down-projected floor, with projectors throwing 1280 x 512 stereo fields at 120 Hz onto each screen. StereoGraphics LCD shutter glasses separated the alternating fields for each eye, switching during the vertical retrace, and the system produced 120 fields per second, updating the whole image at 60 Hz.[6] The authors listed "the lag inherent in common green video projector tubes" among the problems they had to solve. The green phosphor in commercially available projection tubes had too long a persistence, so a user saw both eye images at once and the stereo effect was lost. Until StereoGraphics supplied P43-coated green tubes by special order, the team ran its 1991 experiments using only blue, red and shades of magenta.[6] Electrohome Projection Systems is listed among the project's equipment suppliers.[6]
Persistence and motion
The persistence of a phosphor is defined as the time required for its luminance output to fall to 10% of its maximum. The Army reference calls it the major factor in the dynamic or temporal response of a phosphor display, and notes that in military aviation a loss of temporal response lowers contrast, especially at high spatial frequencies, so fine details such as wires or tree branches can disappear from the image.[1]
On a typical raster-scan CRT, a single frame produces a pulse of light that rises to maximum luminance almost immediately after the frame starts and decays to nearly zero within a few milliseconds; a small phosphor glow can remain at the start of the next frame.[3] Ghodrati and colleagues note that the phosphor decay combined with the rapid scanning means a CRT cannot deliver continuous luminance, so its light flickers at the frame rate.[7] LCD panels, by contrast, are sample-and-hold displays that keep each frame steady until the next one. Elze and Tanner describe motion blur as a well-known and unavoidable side effect of sample-and-hold displays, with the amount of blur set by the frame rate and the hold time.[3] CRTs have their own temporal artifact: a study summarized by Elze and Tanner found considerable visible persistence on a CRT for white stimuli on a black background.[3]
The same trade-off appears in VR headsets as persistence. When Oculus showed its Crystal Cove prototype at CES in January 2014, Road to VR explained that a full-persistence display has its pixels lit all the time, while the prototype's low-persistence technique lights the pixels only when the scene orientation is correct and goes dark immediately afterward.[14] See low persistence and display motion blur for how current headsets handle it.
See also
References
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 Clarence E. Rash, Michael B. Russo, Tomasz R. Letowski, Elmar T. Schmeisser (eds.) (2009). "Helmet-Mounted Displays: Sensation, Perception and Cognition Issues". U.S. Army Aeromedical Research Laboratory, Fort Rucker, Alabama (DTIC report ADA522022). https://archive.org/details/DTIC_ADA522022. Retrieved 2026-10-21.
- ↑ 2.0 2.1 "Karl Braun". Engineering and Technology History Wiki. IEEE History Center. https://ethw.org/Karl_Braun. Retrieved 2026-10-21.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Tobias Elze, Thomas G. Tanner (2012-09-11). "Temporal Properties of Liquid Crystal Displays: Implications for Vision Science Experiments". PLoS ONE, vol. 7, no. 9, e44048. doi:10.1371/journal.pone.0044048. https://pmc.ncbi.nlm.nih.gov/articles/PMC3439495/. Retrieved 2026-10-21.
- ↑ 4.0 4.1 4.2 4.3 4.4 Ivan E. Sutherland (1968-12). "A head-mounted three dimensional display". Proceedings of the AFIPS Fall Joint Computer Conference, Part I (AFIPS '68), pp. 757-764. ACM. doi:10.1145/1476589.1476686. https://www.mmi.ifi.lmu.de/lehre/ss09/ar/p757-sutherland.pdf. Retrieved 2026-10-21.
- ↑ 5.0 5.1 Jannick Rolland, Hong Hua (2005). "Head-Mounted Display Systems". Encyclopedia of Optical Engineering. Taylor & Francis. https://api.creol.ucf.edu/Publications/1519.pdf. Retrieved 2026-10-21.
- ↑ 6.0 6.1 6.2 6.3 Carolina Cruz-Neira, Daniel J. Sandin, Thomas A. DeFanti (1993). "Surround-Screen Projection-Based Virtual Reality: The Design and Implementation of the CAVE". Proceedings of SIGGRAPH 93, pp. 135-142. ACM. doi:10.1145/166117.166134. https://www.evl.uic.edu/documents/siggraph93-cave-cruz-neira.pdf. Retrieved 2026-10-21.
- ↑ 7.0 7.1 7.2 7.3 7.4 7.5 Masoud Ghodrati, Adam P. Morris, Nicholas S. C. Price (2015-03-23). "The (un)suitability of modern liquid crystal displays (LCDs) for vision research". Frontiers in Psychology, vol. 6, article 303. doi:10.3389/fpsyg.2015.00303. https://pmc.ncbi.nlm.nih.gov/articles/PMC4369646/. Retrieved 2026-10-21.
- ↑ Ferdinand Braun (1897). "Ueber ein Verfahren zur Demonstration und zum Studium des zeitlichen Verlaufes variabler Ströme". Annalen der Physik, vol. 296, no. 3, pp. 552-559. doi:10.1002/andp.18972960313. https://doi.org/10.1002/andp.18972960313. Retrieved 2026-10-21.
- ↑ Morton L. Heilig (1960-10-04). "US2955156A: Stereoscopic-television apparatus for individual use". Google Patents. https://patents.google.com/patent/US2955156A/en. Retrieved 2026-10-21.
- ↑ Kent Bye (2015-11-17). "50 Years in VR: Tom Furness' Journey from Making More Lethal Pilots to Non-profit Learning". Road to VR. https://www.roadtovr.com/50-years-vr-tom-furness-super-cockpit-virtual-retinal-display-hit-lab-virtual-world-society/. Retrieved 2026-10-21.
- ↑ I. E. McDowall, M. Bolas, S. Pieper, S. S. Fisher, J. Humphries (1990). "Implementation and integration of a counterbalanced CRT-based stereoscopic display for interactive viewpoint control in virtual environment applications". Proceedings of SPIE, vol. 1256, Stereoscopic Displays and Applications. NASA Technical Reports Server. doi:10.1117/12.19898. https://ntrs.nasa.gov/citations/19910056873. Retrieved 2026-10-21.
- ↑ "Company Profile: Virtual Research Systems, Inc.". Virtual Research Systems (archived 1998-06-10). 1998. https://web.archive.org/web/19980610191600/http://virtualresearch.com:80/company.html. Retrieved 2026-10-21.
- ↑ "Virtual Research Systems FS5 Head Mount Display". Virtual Research Systems (archived 1997-06-28). 1997. https://web.archive.org/web/19970628210202/http://www.virtualresearch.com/fs5.html. Retrieved 2026-10-21.
- ↑ Ben Lang (2014-01-10). "Palmer Luckey and Nate Mitchell Interview: Low Persistence 'Fundamentally Changes the Oculus Rift Experience'". Road to VR. https://roadtovr.com/ces-2014-oculus-rift-crystal-cove-prototype-palmer-luckey-nate-mitchell-low-persistence-positional-tracking-interview-video/. Retrieved 2026-10-21.