This calculator can be used to estimate various combinations of components in a projector.
The units can be changed by clicking on them. (mm, M, inches and Feet)
To find a certain value click on the checkbox next to the textbox and it will turn green. By changing one of the other textboxes the green selected textbox will display the result. For example if you have a 15” LCD and a 320mm FL triplet and you want to get a 100” screen then click the screen distance checkbox to turn it green then change the screen size to 100”. The green screen distance textbox will give you the required distance that the triplet will need to be from the screen. Or you might want to see how much bigger a 17” LCD would make the screen than a 15” LCD at the same distance. Select the screen size checkbox and then change the LCD size from 15” to 17”.

The FL of the triplet as a rule of thumb can be considered at the centre of the lens but this is not always the case. It all depends on the various elements in the triplet; it could be closer to the rear or closer to the front of the lens.

The value that is entered in the LCD size textbox will be the same type as the screen size textbox or vis versa. For example if you enter the LCD diagonal the result in the screen size textbox will also be diagonal. If you enter the LCD width then the screen textbox will also be width.

Warnings and messages:
If you enter a value in a textbox that can’t be used by the calculator then all other textboxes will turn red to indicate that a result is not possible. For example a zero value will cause an error. A message will pop up in the bottom right corner to indicate other types of errors. If in split then entering a value into the triplet FL that is less then the gap will cause an error because the triplet can’t be moved any closer than the physical position of the front fresnel.
You will probably notice that the labels above each textbox may sometimes turn red; this is just a warning that the value in that particular textbox is larger than what is being displayed. Use the arrow keys if you want to see the entire value.

Let me know if you find any bugs.
Please enjoy.

DJ

Bugs:
If you get a “Runtime Error ‘13’ Type Mismatch” message you will need to go into Regional And Language options in XP and choose English

Hi

I am unable to download the calculator. Getting a files moved error message.

gvr

The LCD distance will never be the same as the triplet’s FL. Read this to understand it a bit better.
The fresnel’s FL on the other hand isn’t matched very well and this will happen with almost all the DIY projectors you’ll see here. With the distances you have used, the correct fresnel FL should really be about 360mm (LCD distance - fresnel gap). It can be fudged though by moving the lamp closer to the rear fresnel so the light will refracted further forward so it can pass through the triplet. This isn’t the best approach but we have to use what we have on hand.

DJ

Our projection lens may have a focal length of 320mm, but the if the lens were only placed at its focal length from the LCD panel we would be focused on infinity, so in practical usage we would place it around ~340mm from the LCD panel as represented by line Z (note this distance will vary with different screen sizes). We leave a gap of ~15mm between the condenser fresnel lens and the LCD panel and a gap of ~20mm between the collector fresnel and the LCD panel. This is desirable because if a fresnel lens is too close to the panel, the grooves on the fresnel lens will be projected too. The larger gap for the collector fresnel is to give some room for keystone tilting if needed. If you have circular lines or Moire patterns in your projection your fresnels are too close to the panel.

What about increasing the Fresnel Gap from 20mm to whatever allows the fresnel's FL to be close enough to the triplet? For instance, instead of 20mm, let it be 60mm or 80mm from the panel? I can't find much on the forums regarding the fresnel gap. Does it degrade image quality when it gets much farther than 20mm?

Obviously this would change the size of the projected screen, but if that wasn't an issue, would changing the fresnel gap work?

Thanks,
brick

if you run this on windows server 2003 or windows vista / longhorn

you will need to place msvbvm50.dll in your window/system32 directory

Anyone using Vista might need to download a copy of Msvbvm50.dll to use this. There are a few places it can be downloaded from or get it from someone with XP. Placing it in the same folder with the FocalCalc exe should work.