Mailbag: Hotspotting

July 3, 2006

Test PatternWhile catching up on some email over the weekend I ran across one from Brent in Columbia SC, Brent writes:

“I’ve heard talk of hotspotting from salesmen and various reviewers when describing certain types of displays. What is this, what does it look like and how can I make sure I avoid it in a new purchase? Thanks”

Thanks for the note Brent; hot-spotting is a term that’s used to describe the effect of getting uneven or pronounced areas of light, in the center of (front or rear) CRT, D-ILA/SXRD, DLP and LCD based projection displays.

Hot-spotting is most often found in front projectors paired with high-gain screens or in older rear-projection televisions that used high contrast masks (or filters) to improve off-axis viewing angles.

Often times the optics in many budget displays aren’t up to par with their light output and cause the light to be projected with poor linearity, resulting in the aforementioned ‘hot spot’ or greater intensity of light in the center of the screen.

With front projection systems the higher the gain of screen used, the more the screen takes on the properties of a mirror, i.e. reflects the light back in-line with the conical light path from the projector, so eliminating hotspotting in front projection systems is a two-fold problem. Often times it’s the combination of screen gain and projector that causes the problem and not just the projector itself.

For example I recently visited a Magnolia Home Theater to audition the Sony VPL-VW100, unfortunately they had opted to use a relatively small, high gain screen in a light controlled room. The result was less than optimal, when I asked why a high-gain screen in a room with the ability to reach near dark, the salesmen replied, ‘we need to turn on the lights occasionally’.

Fortunately hotspotting has become much less common in rear projection displays. I have to assume this is due partly in fact to manufacturers realizing that the ‘we can do off-axis viewing better than you’ nonsense of years past was counter productive.

As for how to best avoid hotspotting in a new display; first be sure to try and audition displays in similar lighting levels as you’ll view them in at home, especially with rear projection based displays. Also avoid high gain screens unless your room conditions absolutely dictate their use.

Or in other words the use of a high gain screen to band-aid a projector with poor contrast isn’t exactly a wise investment. You’d be better off to divert the additional cost of the high gain screen, back into a projector with a better ANSI contrast ratio. Of course that is unless as mentioned your room absolutely requires the use of a high gain screen.

Posted by Bryan Greenway | | Filed Under Display Technology