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Several threads have refered to adding extra glass sheets to protect the fresnel lenses from damage or dust. A couple of threads have added these after making and using their projectors only to find a degradation of the image brightness. Others see little difference. Yet others claim a vastly improved image after removing extra glass.
I have spent an afternoon researching this topic to learn the truth.
The transmissability of glass or plastic is determined by several factors. The most important are clarity of the basic material, impurities in the material, and reflectivity. Wavelength of the light is also important as different types of glass or plastic will pass different parts of the spectrum unequally.
Clarity and impurities: Essentially glass and plastic are not totally clear. Even our atmosphere is not totally clear. The transmission of light will lessen with the distance it is required to travel through ANY material as the photons fall prey to either the inherent clarity or the impurities in the material and be absorbed. We can see this simply by looking at the EDGE of a glass panel. As the lower energy wavelengths are absorbed by impurities in the glass or the glass itself, the light travelling through the length or width of the glass tends toward the higher energy green and blue. Eventually, even these shorter wavelengths will be absorbed. Therefore the thicker the glass the less light can be transmitted through it.
Standard window glass (whether tempered or not) will absorb approximately .6% per millimeter of the light transmitted through it. Transparent Lexan or Acrylic plastic has about the same absorbtion pattern as window glass. A standard window pane (~3mm, ~1/8") will absorb about 2% of the light passing through it!
Reflectivity: polished surfaces do not let all of the light in...or out! The highly polished surfaces of the glass WILL reflect a percentage of all light that falls on them. In sheet glass, there are always TWO surfaces... one on each side... and each will reflect some of the light that strikes them. Untreated glass surfaces will reflect about 4.5% of the light that strikes perpendicular to the surface... and it gets worse the greater the angle of incidence until 100% is reached approaching an angle of incidence of 90%.
Special optical coatings can be applied to glass (or plastic) to reduce the reflectivity. The most efficient of these will reduce the reflective index to 1.4%... but we generally don't have access that kind of glass at anything like a reasonable price.
Each surface of a standard uncoated window glass (tempered or not) or Lexan pane will reflect approximately 4.5% - 5% of the light striking that surface depending on the angle of incidence. Since each piece of glass has TWO surfaces, approximately 9% - 10% of the light striking the glass WILL be reflected!
Wavelength: Glass is generally better at transmitting high energy light than low energy light... and that is apparent when one looks at the ability of the light to transmit various colors of visible light. The first chart below shows the ability of glass to transmit the solar spectrum. The second chart shows various types of glass and/or coated glass transmission or transparency.
The charts go from Ultra-violet at the left to Infra-red at the right. The higher the graphline (Y axis) the greater the transmission or transparency. When the graphlines intersect the base line at the bottom it means the glass is opaque (non-transmitting) of those wavelengths.
We can see that all glasses are essentially opaque to most of the Ultra-violet light spectrum, especially to the higher energy Ultra-violets that can do the most damage. Unfortunately, it is also obvious that glass is almost as transparent to Infra-red as it is to visible light. Also unfortunately, to control that Infra-red, coatings to filter out Infra-red either impact the visible light spectrum unequally or cause a 50% reduction in the transmission of visible light! This is why cooling our LCD panels is so important!
Looking at the wavelength charts for Lexan and other plastics we find a similar pattern: blocking of Ultra-violet and transparency to visible light spectrum along with Infra-red (that is why plastics and glass have similar usages in greenhouses... they both let in short Infra-red waves and block long Infra-red waves.)
So, exactly how much light is lost at each additional glass or Lexan panel added to our projectors? Brainchild's original design includes four (4) panels: one tempered glass heat shield, the Field Fresnel Lens, the LCD screen, and the Collector Fresnel Lens. Brain estmates that about 80% of the light that falls on the LCD screen is absorbed. This could be because there are actually several layers in the LCD, each with surfaces and transmission issues... as well as two polarizing filters that cut down quite a bit of the light merely be passing only light that is oriented in a specific angle. So let's use that 80% estimate. That gives the LCD screen about a 20% efficiency. I will also assume that the optics that Brain sells are coated to reduce reflection. We will assume a fresh 40,000 lumen light source with a Norpro bowl reflector, delivering 40% of its output to the first glass element.
From the research done above, each NON-COATED panel will reflect ~10% (5% x 2 surfaces) and absorb 2% (3mm thickness x .6%/1mm)... meaning an ~88% efficiency. Each COATED panel will reflect ~3% and absorb 2% (3mm thickness x .6%/1mm) for a 95% efficiency.
Light striking first element (tempered glass) = 40,000 L X 40% = ~16,000 Lumen
Light leaving first element (tempered glass) = 16,000 L X 88% = ~14,080 Lumen
Light leaving second element (Fresnel Lens) = 14,080 L X 95% = ~13,376 Lumen
Light leaving third element (LCD Screen) = 13,376 X 20% = ~2675 Lumen
Light leaving fourth element (Fresnel Lens) = 2675 X 95% = ~2541 Lumen
The next element is the Triplet lens... which has six coated surfaces (3 lenses X 2 surfaces) and about 25mm of glass which probably gives it an efficiency of about 76% (9% loss from surfaces plus 15 % loss from glass thickness).
Light leaving the fifth element (Triplet Lens) = 2541 X 76% = ~1931 Lumen
This gives us a theoretical Lumen level available of approximately 1931 Lumen. Pretty impressive. (Without the Norpro bowl, it is probably about 40% less than than that)
Now, let's add the extra window glass designed to protect the fresnels from dust or to stiffen them.
Light striking first element (tempered glass) = 40,000 L X 40% = ~16,000 Lumen
Light leaving first element (tempered glass) = 16,000 L X 88% = ~14,080 Lumen
Light leaving second element (Fresnel Lens) = 14,080 L X 95% = ~13,376 Lumen
Light leaving first added window glass protector/stiffener = 13,376 X 88% = ~11,771 Lumen
Light leaving third element (LCD Screen) = 11,771 X 20% = ~2354 Lumen
Light leaving second added window glass protector/stiffener = 2354 X 88% = ~2072 Lumen
Light leaving fourth element (Fresnel Lens) = 2072 X 95% = ~1968 Lumen
Light leaving the fifth element (Triplet Lens) = 1968 X 76% = ~1496 Lumen
So, with just two additional panes of window glass we have a theoretical output of only approximately 1496 Lumen which is about 77% of the basic model... or about a 22% loss.
Adding two more as some have suggested with the tempered glass separated a distance from the first Fresnel lens:
Light striking first element (tempered glass) = 40,000 L X 40% = ~16,000 Lumen
Light leaving first element (tempered glass) = 16,000 L X 88% = ~14,080 Lumen
Light leaving the first added window glass protection/stiffener = ~13,024 Lumen
Light leaving second element (Fresnel Lens) = 13,024 L X 95% = ~12,373 Lumen
Light leaving second added window glass protector/stiffener = 12,373 X 88% = ~10,888 Lumen
Light leaving third element (LCD Screen) = 10,888 X 20% = ~2178 Lumen
Light leaving third added window glass protector/stiffener = 2178 X 88% = ~1917 Lumen
Light leaving fourth element (Fresnel Lens) = 1917 X 95% = ~1821 Lumen
Light leaving fourth added window glass protector/stiffener = 1821 X 88% = ~1602 Lumen
Light leaving the fifth element (Triplet Lens) = 1602 X 76% = ~1218 Lumen
The result of stiffening by sandwiching the Fresnelsbetween two pieces of glass is a reduced output of only approximately 1218 Lumen. This is a significant reduction of 37% of what should be possible. Without the Norpro bowl, the loss of light would be striking! The output would be only about 700 Lumen.
Again, Brainchild's original plans and optic design is best (so long as we add the Norpro bowl!).
Recap of results
Original Brainchild design w/o Norpro reflector = 1160 Lumen,
Original Brainchild design w/ Norpro = ~1931 Lumen
Modified design with Norpro and 2 Fresnel stiffeners w/o Norpro = ~890 Lumen,
Modified design with Norpro and 2 Fresnel stiffeners w/ Norpro= ~1496 Lumen
Modified design with Norpro and 4 sandwiched Fresnels w/o Norpro = ~700 Lumen,
Modified design with Norpro and 4 sandwiched Fresnels w/ Norpro = ~1218 Lumen
The only conclusion I can come to is use the Norpro reflector and no extra glass.
