Film-Tech Forum ARCHIVE: Color temp of bulb and its effect on focus & sharpness?

This is topic Color temp of bulb and its effect on focus & sharpness? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Alan Gouger (Member # 543) on 07-19-2005, 11:27 AM:

Maybe you guys can shed some light on this.

Im running a portable projector backed by a quartz halogen bulb. Plenty bright for my 8 foot wide scope image.
This bulb appears to provide a much whiter light then the 1000 watt incandescent bulb that came with the projector.

I also have a HIT bulb of same wattage but this bulb is very similar to xenon in character and requiers an ignitor and seperate power supply. The light from this bulb is very bright & very white, higher in color temp to the halogen. Has that WOW factor.

The HTI bulb yields a much sharper image with more detail then the halogen. Far more. Focus is also much easier on the eyes to achive and maintain verses the halogen which always appears softer even when perfect focused is achived.

Is the visual improvement in detail & sharpness caused by the color temp differences between the bulbs or is it from the HTI being brighter.

Another words if its because of brightness then would a brighter halogen yield the same result or is the color temp the reason for the visual improvement.

Thanks!!

Posted by Paul Mayer (Member # 355) on 07-19-2005, 12:55 PM:

Just a guess here...

The halogen lamp probably has a light source (filament) much bigger than than the almost point source (short arc?) of the HTI. The reflector was probably designed to take advantage of a point source. Any light eminating away from the expected point source (as a relatively large filament array will do) will not be focused correctly by the reflector. All those incorrectly focused light rays will play havoc with image focus at the aperture.

It's almost the same affect as having a lamphouse not properly aligned with the projection axis. In addition to illumination uniformity issues, there are picture focus issues when the lamphouse is not properly aligned to the projector.

I don't think color temperature has any affect this focus and sharpness issue.

Up to a point, more light is certainly better. Colors and contrast will look murky when too little light is used. Murky contrast can affect how people perceive image sharpness.

That's my guess anyway.

Posted by Alan Gouger (Member # 543) on 07-20-2005, 10:22 AM:

Thanks Paul. That makes sense.

Posted by John Pytlak (Member # 331) on 07-21-2005, 09:54 AM:

Generally a tungsten or tungsten halogen lamp (lamps with filaments) will emit much more infrared energy than an arc lamp having the equivalent light output. So more care is needed to manage heat issues:

Heat Damage Part 1

Heat Damage Part 2