Hotspotting Issues

[dracul]

I just came from a lighting store where they hooked up an Osram 400W hqi Powerstar for me to test. I put a 9x12 inch white sheet of paper at 9 inches away and took light readings with my footcandle meter to see how even the light across a 12 inch wide area(15 inch diagnol lcd)
The difference between the center and the sides was 1/4 light loss. I made a deal with the light shop owner that I could take my projector in tomorrow and test it with his bulb before buying it but my gut tells me I am still going to get the some hotspotting.
Which means my troubles are not with my lamp as I was thinking.
So is my problem with my fresnels or fresnel placement? I already tried the first fresnel making sure its at its FL which is 9 inches. Any other distance makes the hotspotting worse. The top fresnel can only go at one distance and that is about 2 inches away from the lcd.

[dracul]

Sorry, correction. MY osram 150watt hqi is not as even as the 400watt version
The 150 watt hqi looses 1/2 the light from the center to the sides
compared to the 400watt which is only 1/4 loss.
The osram hqi 400watt puts out more than 2 stops or 4 times more light than the 150hqi. That should make it a 600watt technically.

[bfourcade]

Hola Dracul,

(te escribo en mi pobre ingles por el resto del foro)

I´ve been reading all your post and I can recomend the lamp I´m using.
It´s a KRC400T/H960/E39 a 400w Metal Halide (Mercurio Halogenado) from General Electric with great results.

This bulb is great because it is 6000k (color temperature) and more than 90 CRI (Color rendering index)

It works very weel in my gear but its really really hot. I´ve solved the hot problems with asbestos (amianto) so the cabinet stay really cold.

Here is the link to the bulb specs. (Si te decides y tienes deficultad para conseguirla en tu ciudad aqui en Córdoba la puedo conseguir en una casa de iluminacion.)

http://catalog.gelighting.com/cgi-bi...6&category=HID

Bernardo

[Hezz]

Dracul,

You may have to split the fresnels and put the LCD between them. Though this may lower your light level somewhat it will improve the light eveness and contrast of the picture. Some LCD's may be more sensitive to the light going through the LCD at an angle and the further you are from the center of the LCd the larger the angle is. This is causing the hot spots assuming that the light engine is aligned properly.

Some LCD have more problems with off angle light. Don't try and get the light even with a light meter because you never will. Just shot for a better subjective picture.

Hezz

[dracul]

Ive tried them split and while it is a better its still not even.
I am not using an lcd yet just a transparency with black 8 point text. On the sides the image is quite a bit dimmer.
I am thinking the fresnels are as good as it gets because the demos of OHPs I saw in the stores were exactly the same.
But apparently everyone is not getting this. Brainchild certainly isnt.

[dracul]

Bernardo you are using wooden box or aluminum.

[bfourcade]

Dracul,
In the first run protoype I´ve was using wood because i´d need be free to move all the components to measure everything before build a beta version.
(Focal lenght of the lenses, try with or without condenser, etc.
In this stage wooden near to the lamp begun to smoke, so I stop (having measured everything) and decided to look for an insulation material.

I´ve found asbestos (amianto) doing a terrific job so I´ve decided and risked to build a beta version in a Pc Cabinet, so in this first version I have a reflector (made from a soup laddle), the big bulb at 1 cm, the condenser lens at .5cm a mirror at 20cm, behind the mirror the ATx Ps, The panel PS and the bulb Balast and starter, all in the cabinet, closed with the fresnell sandwiched inbetween 2 x 6mm glasses. (plus a 3" fan in one side that pull the bulb heat out off the case)
So its´is a very compact OHP style proyector.

Asbestos really aisle heat. For you to see, the bulb is 2cm of the front side of the cabinet, the front side is PLASTIC made, so between the lamp (actually the reflector) and the plastic I´ve putted 3 layers of 3mm asbestos sheets. Running for 3 hours you can touch the plastic and its only warm as a result of the integral heating of the struture not as result of direct heat radiaton from the lamp. The structure heat (nothing to worry about)) is because there are some points I´have not covered with asbestos (obviusly the mirror) that conduct some heat to the structure but mainly because Ive not made any air vents for input fresh air (is the next step) so it will runs completely cold.

The only problem I see with my big lamp is that. It´s too big (with the mog base it´s 29cm long) . If I´d have an smaller bulb I could put all the things (LCD and proyection lens included) inside a MidTower PC cabinet with the proyection lens in the plastic front side getting a pro look in a compact case.


I hope it helps,

Bernardo.

[Hezz]

Dracul,

It sounds like nothing is wrong with your system. When you put in an LCD it will diffuse the light more than a transparancy so the uneven light will be less noticable. Two things you can do to get the light more even. One you can move the condenser lens or the light back a small amount which will put the light slightly out of focus. This will blur some of the uneveness.

The second method should be more effective but requires some more work. You can collumate the light before sending it into the condenser lens with a trapazoidal reflective light pipe. Chech out the light engine on my large metal frame projector design. It shows the light collumator inside the larger light pipe.

Hezz

[dracul]

Hezz,

I dont have a condenser. I would really prefer to keep the fresnels behind the lcd and this results in uneven lighting due to the narrow spot of light that comes out the other end or the 13 inch FL fresnel. So it is very obvious the beam does not cover the entire lcd with this method. What else can I use to diffuse the light right now and test? Can I use some kind of semi opaque sheet . That is a very good point. It may not be a little diffusion it may be a lotttttttttt in which case this will make a BIG difference.
Has anyone tested their box without the lcd or a transparency?
The lcd sucks a lot of light and its got a lot of little pixels that I am sure has a significant effect. Thanks for this point Hezz. What can I try before buying the lcd though?

[dracul]

Can someone please do this test for me if possible. Replace your lcd with a 9 x12 inch transparency with black text and tell me if you get uneven lighting. I would appreciate it since it would confirm the lcd does the missing work that I require.