Hello All DIY Gurus
is it necessary to have 2 fresnels?
i burnt my first frensel want to buy another, on "ebay.in" i have some sellers saying they have frensel of power 6X and 10X
what does it mean?
if i buy two fresenl do i need both of them same? like two 6X or two 10X
or
will one 6X and one 10X will do?
pls ignore my lack of knowledge
and pls help
PD
is it necessary to have 2 fresnels?
i burnt my first frensel want to buy another, on "ebay.in" i have some sellers saying they have frensel of power 6X and 10X
what does it mean?
if i buy two fresenl do i need both of them same? like two 6X or two 10X
or
will one 6X and one 10X will do?
pls ignore my lack of knowledge
and pls help
PD
Hi, paragdesh, the reason for two Fresnels, (correct me if i'm wrong, Mods!) is that the one before the LCD spreads the light from your lamp across the LCD more evenly than without one, and the second is there to direct the light from the LCD towards the PJ lens.
A friend of mine is getting away with only using the second Fresnel to project a very good image, but he is using MH bulbs, dont know if this makes a difference, but it is very bright.
you DO need the front Fresnel, as without it, you, (well, me,) only get the centre of the LCD projected, a small circle, just.
Not sure how to work out which Fresnels you need, maybe someone else can help.
Have only heard of them being described in focal lengths.
Hope this is of some help!
D2D
😀
My DIY crossflow fan!
DIY cinema forum, new members welcome! UK based!
A friend of mine is getting away with only using the second Fresnel to project a very good image, but he is using MH bulbs, dont know if this makes a difference, but it is very bright.
you DO need the front Fresnel, as without it, you, (well, me,) only get the centre of the LCD projected, a small circle, just.
Not sure how to work out which Fresnels you need, maybe someone else can help.
Have only heard of them being described in focal lengths.
Hope this is of some help!
D2D
😀
My DIY crossflow fan!
DIY cinema forum, new members welcome! UK based!
I Think i got it
Hello D2D and All Gurus
ok now after searching on google,forum's and D2D's reply i am more clear and here i ma uploading a diagram , i suppose i am clear on this, this means single fresnel wont work?
but how come then the OHP has only one fresnel?
i had burnt my OHP's frensel so i broke it open ...and it was ripped in two with both sides having the grooves, does that mean ohp uses 2 fresnel?
this is not corcerned with forum, but if anybody can put some light on this topic?
Parag
Hello D2D and All Gurus
ok now after searching on google,forum's and D2D's reply i am more clear and here i ma uploading a diagram , i suppose i am clear on this, this means single fresnel wont work?
but how come then the OHP has only one fresnel?
i had burnt my OHP's frensel so i broke it open ...and it was ripped in two with both sides having the grooves, does that mean ohp uses 2 fresnel?
this is not corcerned with forum, but if anybody can put some light on this topic?
Parag
Yes, you have got it, there is actually two Fresnels in "most?" OHP's they are usually stuck together, with the grooves facing each other.
It is a bit risky splitting fresnels, but it can be done, if you are careful.
As you have damaged yours, you need to get new ones. 🙁
As for this not being relevent? it surely is, as lots of people on this forum, and others, use Fresnels, split or unsplit.
The split fresnel giving the best results. (IMO)
Your pic is a good representation of how they work!
OHP's have two Fresnels, stuck together, as you have found out!
D2D
🙂
DIYCINEMA
It is a bit risky splitting fresnels, but it can be done, if you are careful.
As you have damaged yours, you need to get new ones. 🙁
As for this not being relevent? it surely is, as lots of people on this forum, and others, use Fresnels, split or unsplit.
The split fresnel giving the best results. (IMO)
Your pic is a good representation of how they work!
OHP's have two Fresnels, stuck together, as you have found out!
D2D
🙂
DIYCINEMA
Ok i found a manufacturer of fresnels for OHP, he agreed to send me the fresnels already split 330 and 220, so my next question is
what is the position of the fresnels
330 faces light source or 220 faces light source
or vice versa?
can nebody help
PD
what is the position of the fresnels
330 faces light source or 220 faces light source
or vice versa?
can nebody help
PD
I believe the reason to use 2 fresnel lens instead of 1 fresnel is that:
If the light from a point light source hit the ridged side of the fresnel, a large amount of light hits on the off-center area of the fresnel will be reflected or even be totally reflected because of the "Critical Angle". However, the parellel light source does not have the problem of uneven reflection loss on the ridged side of the fresnel. Also, the smooth side of the fresnel does not cause significant uneven reflection problem, and this is why to face the smooth side of fresnel toward the bulb.
The function of the first fresnel is to make the point light source into parellel light source, so to avoid the "total reflection" on the ridge side of the second fresnel. The function of the second fresnel is to converge the parellel light send the light to the projecting lens.
If the light from a point light source hit the ridged side of the fresnel, a large amount of light hits on the off-center area of the fresnel will be reflected or even be totally reflected because of the "Critical Angle". However, the parellel light source does not have the problem of uneven reflection loss on the ridged side of the fresnel. Also, the smooth side of the fresnel does not cause significant uneven reflection problem, and this is why to face the smooth side of fresnel toward the bulb.
The function of the first fresnel is to make the point light source into parellel light source, so to avoid the "total reflection" on the ridge side of the second fresnel. The function of the second fresnel is to converge the parellel light send the light to the projecting lens.
Hey man thnks
but now i have another confusion
some say the rigid surface faces bulb some say smooth surface
help me gurus!
light source -> rigid|plane -> lcd -> plane|rigid -> proj'n lens
(fresnel) (fresnel)
or
light source -> plane|rigid -> lcd -> rigid|plane -> proj'n lens
(fresnel) (fresnel)
PD
but now i have another confusion
some say the rigid surface faces bulb some say smooth surface
help me gurus!
light source -> rigid|plane -> lcd -> plane|rigid -> proj'n lens
(fresnel) (fresnel)
or
light source -> plane|rigid -> lcd -> rigid|plane -> proj'n lens
(fresnel) (fresnel)
PD
rigid=screw thread?
if yes
light source -> plane|rigid -> lcd -> rigid|plane -> proj'n lens
(fresnel) (fresnel)
if yes
light source -> plane|rigid -> lcd -> rigid|plane -> proj'n lens
(fresnel) (fresnel)
I say the rigid side never face the light source, unless the light source is parallel light source. We probably need some professional to settle this issue.
desperadok said:
I agree, most fresnels i have seen, have the smooth side facing the light, and if you are using the split fresnel method, the second one has the ridged side towards the LCD/light.
I think that the smooth side facing the lamp, the fresnel spreads, (defuses?) the light entering it into paralell rays, while the other way, (ridges facing lamp,) it focuses? the light to a point, i.e. the projection lens.
If i remove my second fresnel from my PJ, only the center of the picture is projected, the paralell light going through the LCD mostly missing the PJ lens. 🙂
So i think that there might be ways of getting away without the first fresnel, IMO the second fresnel is necessary to direct the image to a smaller size to fit the PJ lens, otherwise you would need a PJ lens that covers the whole LCD area, to collect the paralell rays leaving the LCD. (like in a slide projector the lens is bigger than the slide)
D2D
🙂
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