Brown or Blue Edge on Discharge Beam

[chrisprescott]

I was centering up the lamp on one of our old 2kw xenon colourarc followspots recently.

 

I noticed that the beam had a brown edge rather than a blue edge as is normal.

 

I am mostly sound not lights, but I did wonder if anyone had any advice about why the edge might be brown rather than blue, and ways of removing this edge to the beam so it is just a pure white circle, or is it always there and I have never noticed before :)

 

If this has been brought up on another thread please do redirect me! I did search but couldn't find any info.

 

Thanks in advance!

[J Pearce]

Brown or blue is to do with which side of the focus point you are and is caused by the lens' refractive index (focusing power) being slightly different at each end of the spectrum. This effect is called chromatic abberation. Cheaper lenses will often have extended areas of brown or blue sometimes even showing a spectrum, better lenses balance the effect and only have a slight tinge. These are often made of low dispersal glass or use a composite lens made of several different glasses, known as an achromatic or apochromatic lens.

 

Careful focussing can minimise the effect, and it is usual to go to the blue side of focus, though some LDs prefer to go to the brown side.

[chrisprescott]

Thanks Jon,

 

I did try to focus but couldn't get it to the blue. I am guessing that it is likely because these are just old optics?

 

I have emailed xenonspot to see if they have a service manual but I am guessing that being from the 80s this is probably unlikely. Does anyone have any advice as to what to check on these?

[itiba]

One way to get rid of the brown edge on a stubborn profile is to add a slither of light diffusion however not ideal if you're looking for a sharp edge.

 

I'd make sure the lenses were clean and see if adjusting the lamp alignment yields any results as that can often change the optical path... good luck!