100 % Brighter 7" with 230mm Objective?

dracul · 16th January 2006, 02:42 AM

I am building a 7 inch lcd projector. At the moment i 'm planning on using the following gear.

220mm first fresnel
320mm top fresnel
320mm objective

What if I had the following equipment?

200mm first fresnel
200mm top fresnel
230mm objective

The projection lens will sit 90mm closer to the top fresnel which should give me almost double the light acording to the inverse square law of light.

Am I right or wrong?

tgreenwood · 16th January 2006, 03:43 AM

The top fresnel simply directs all of the light into your objective/projection lens.

The only effect of changing the top fresnel and the objective lens to a shorter focal length will be to make your enclosure shorter and the image bigger at the same throw distance.

Tgreenwood

dracul · 16th January 2006, 03:56 AM

Ok lets say that you place the triplet at 50 feet and there is a fresnel set to direct all the light from the lamp into the triplet still.

So at 50 feet the back of the triplet should receive the same amount of light as at 20 inches away?

That would be true if there didn't exist something called the inverse square law of light. Every 25% increase in distance deminishes light intensity by half.

Camman · 16th January 2006, 11:40 AM

on paper 100% yes. In the real world, it will look a whole ****load brighter, but I doubt you will truly get double the intensity. If it were me I would go ahead and do it, the only difference is going to be your image size. With the shortal focal length of the objective your image will be much larger at the same throw.

pinkmouse · 16th January 2006, 11:46 AM

Inverse Square law only works for direct radiation, if you modify the light with lenses or reflectors it's no longer valid.

dj_holmes · 19th January 2006, 01:10 AM

NEED RESULTS. NEED FRESNEL. NEED DECISION. EXECUTIVE DECISION

tgreenwood · 19th January 2006, 03:06 AM

Inverse square law

Check out the link above.

Especially the part that says "The energy twice as far from the source is spread over four times the area, hence one-fourth the intensity."

Whatmo · 19th January 2006, 05:04 AM

The distance they are refering to here is from your lens to the surface you are projecting onto.
The actual movement of the light through air does not decrease its brightness a noticeable amount. Think of a laser, it is just as bright no matter how far you shine it.

dracul · 19th January 2006, 11:22 PM

Quote:

The actual movement of the light through air does not decrease its brightness a noticeable amount

You lost me. Through what medium do you suppose light travels between the lamp and the projection lens. Is that not air also.

Whatmo · 20th January 2006, 05:59 AM

Yes it is air and that why this distance doesn't matter. As long as you manage to focus your rays of light from bulb into the objective it doesn't matter how far away it is.

The Inverse square law really just says that if the objective lens is farther from the surface being projected onto it will be less bright. This is because the image will be larger. Not because it is travelling through more air.

I hope this helps

16th January 2006, 02:42 AM	#1
dracul diyAudio Member Join Date: Jul 2003 Location: Brazil	100 % Brighter 7" with 230mm Objective? I am building a 7 inch lcd projector. At the moment i 'm planning on using the following gear. 220mm first fresnel 320mm top fresnel 320mm objective What if I had the following equipment? 200mm first fresnel 200mm top fresnel 230mm objective The projection lens will sit 90mm closer to the top fresnel which should give me almost double the light acording to the inverse square law of light. Am I right or wrong? __________________ 1900x1200 All the way baby!!!

16th January 2006, 03:56 AM	#3
dracul diyAudio Member Join Date: Jul 2003 Location: Brazil	Ok lets say that you place the triplet at 50 feet and there is a fresnel set to direct all the light from the lamp into the triplet still. So at 50 feet the back of the triplet should receive the same amount of light as at 20 inches away? That would be true if there didn't exist something called the inverse square law of light. Every 25% increase in distance deminishes light intensity by half. __________________ 1900x1200 All the way baby!!!

16th January 2006, 11:46 AM	#5
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	Inverse Square law only works for direct radiation, if you modify the light with lenses or reflectors it's no longer valid. __________________ Al The miracle is not to fly in the air, or to walk on the water; but to walk on the earth. Chinese Proverb

19th January 2006, 03:06 AM	#7
tgreenwood diyAudio Member Join Date: Mar 2004 Location: Texas	clarification..... Inverse square law Check out the link above. Especially the part that says "The energy twice as far from the source is spread over four times the area, hence one-fourth the intensity."

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16th January 2006, 03:43 AM	#2
tgreenwood diyAudio Member Join Date: Mar 2004 Location: Texas	The top fresnel simply directs all of the light into your objective/projection lens. The only effect of changing the top fresnel and the objective lens to a shorter focal length will be to make your enclosure shorter and the image bigger at the same throw distance. Tgreenwood

16th January 2006, 11:40 AM	#4
Camman diyAudio Member Join Date: Dec 2005 Location: las Vegas	on paper 100% yes. In the real world, it will look a whole ****load brighter, but I doubt you will truly get double the intensity. If it were me I would go ahead and do it, the only difference is going to be your image size. With the shortal focal length of the objective your image will be much larger at the same throw.

19th January 2006, 01:10 AM	#6
dj_holmes diyAudio Member Join Date: Nov 2004 Location: london	NEED RESULTS. NEED FRESNEL. NEED DECISION. EXECUTIVE DECISION

19th January 2006, 05:04 AM	#8
Whatmo diyAudio Member Join Date: Aug 2005	The distance they are refering to here is from your lens to the surface you are projecting onto. The actual movement of the light through air does not decrease its brightness a noticeable amount. Think of a laser, it is just as bright no matter how far you shine it.

20th January 2006, 05:59 AM	#10
Whatmo diyAudio Member Join Date: Aug 2005	Yes it is air and that why this distance doesn't matter. As long as you manage to focus your rays of light from bulb into the objective it doesn't matter how far away it is. The Inverse square law really just says that if the objective lens is farther from the surface being projected onto it will be less bright. This is because the image will be larger. Not because it is travelling through more air. I hope this helps

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