[visionlist] Re: CRT Vs LCD

mailinglists at tobias-elze.de mailinglists at tobias-elze.de
Wed Mar 5 00:14:31 GMT 2008


Hi Ali,

> There are very fast LCDs. Did anyone try them?
> 
> Note: Example, www.viewsonic.com/pdf/wp_4msResponseTime050205.pdf

I didn't try out any of their monitors, but we have measured response times of more than a dozen LCD monitors, and having compared my results to the manufacturers' specification makes me doubt the results shown in the paper you cite there. Some reasons for this:

(1) They don't specify the monitor settings used for the measurements. The response times vary for different settings, and manufacturers tend to choose their measurement settings not according to sensible values for someone working with the monitor but to receive shortest response times.

(2) They don't specify their measurement and data analysis procedure. Did they measure from the 10% to the 90% value of the transition, or did they try to find the saturation points of the two plateaus (which makes an enormous difference), or did they do something completely different? Moreover, how did they filter the backlight effects, smooth the data etc.? Why don't they cite any measurement standard according to which they had been working?

(3) If they really receive response times as short as they show in Fig. 5, they must use heavy overdrive. If so, their Fig. 1 definitely can't be from one of their monitors: An overdrive optical wave transform is characterized by an overshoot of the luminance in the rising transition, so that the luminance exceeds the 100%-level and then approaches asymptotically to it from above. In the figure shown, it approaches from below. 

The last point matters if you want to do vision science with such a monitor: Just imagine you want to change a stimulus luminance from one frame to the other from a value v1 to a value v2. What you get in an overdrive monitor is a sharp transition from v1 to v3 = v2+x, and then a slow transition from v3 to v2. The manufacturers usually calculate the time from v1 until the luminance curve first gets to the 100% (or usually 90%) Level of v2, but not the time it takes the signal to _saturate_ at v2 after reaching v3. And without you noticing this, the stimulus is presented for a short time brighter than you had actually expected knowing your "target luminance" v2 only.


Best,

Tobias.



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