[visionlist] Update - Accurate timing of projected images

Stuart Jackson stuart.jackson at ucdconnect.ie
Wed Jun 28 10:22:39 GMT 2006

Dear all,

Recently I sought information on the the kinds of projectors used for 
displaying images where accurate timing is an issue. Here's a summary 
of the information i've gathered on the topic, from responses i 
received and elsewhere. Thanks again, hope the info. is helpful: 

1) LCD projectors can have slow response times, as the colour wheel is 
not synched to the input. Also they have lower contrast than other 
types of system, and at some magnifications grid lines can be seen 
between pixels. These mightn't be appropriate where accurate stimulus 
timing is required, particularly when explicit control over the 
vertical refresh rate is desired. However, some have used 2 LCD 
projectors in combination to synchronise two separate presentations. 
2) Some DLP systems can contain motion artifacts due to a dithering 
process used to increase resolution. Single panel systems may also 
contain color-breakup artifacts due to having to sequence between r, g, 
and b. A newer projection technology, LCoS, has very high levels of 
contrast and can operate at high refresh rates. However, like some 
LCD/DLP systems, I'm not sure if any currently available systems offer 
the user explicit control over the vertical refresh rate. The 
specifications for many systems claim that vertical refresh rates 
between, for example, 50-85Hz (LCD/DLP) and 50-120Hz (LCoS), can be 
achieved. However, some sources suggest that in reality this isn't the 
case and that the projectors typically display at the lower end (e.g. 
50/60Hz), even if the input signal is at a higher rate.  
3) CRT projectors may be the most appropriate where stimulus timing is 
important, though cost can be prohibitive. Some systems can apparently 
project at frequencies between 30-200Hz+. When the display rate needs 
to be the same as the input signal rate, I think the important thing to 
look for is a reference to something like an 'auto-lock' feature, 
whereby the projector takes on whatever rate is fed into it.      


Stuart Jackson
School of Computer Science and Informatics
University College Dublin

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