Originally posted on 08-24-2004 (may be edited to clarify or add information, or other proof-reading fixes.
Soda Lime or regular float glass, are all basically normal house windows (green when you look edge on).
Optiwhite, crown glass, borofloat, pyrex, and borosilicate are all more clear. It is also known as waterwhite glass.
Getting it clear is only half the battle, typically clear will get you to 91.6% of the light passing through, the next step is to go for doublesided AR (anti-reflecitive) coating. Depending on thicknesses, AR coated window glass (which is green), passes more light than the clear glasses without AR coating.
Okay, so, what type of glass do you have in your hand? Turn it on edge, and look from the edge to inside the glass. If it is green, you have soda lime float glass. (otherwise known as window glass).
Soda lime float glass (a.k.a. window glass) that is often used in flashlights can cause a slight shift in emitted light towards the green portion of the spectrum. How far it shifts will depend on the glass thickness and the quality of the glass.
I should run a test one of these days to see exactly how far the soda lime float glass actually shifts the light color, on top of dropping the light output level.
Without AR coating, you have a 3-5% drop in light output (typically 4%) for each face, for a total of 6-8% loss, on top of 30 - 15% losses for common flashlight reflectors(usually more towards 30%, incandescent bulbs often see typical reflector losses of 35%). You can drop the reflector loss to a total reflector loss of 2 - 15% when you go to lab or telescope quality reflector coatings, instead of the high profit margin reflector coatings you typically find in flashlights.
You can make an aluminum reflector from 100% pure aluminum (the normal aluminum you get is not pure but an alloy), and get the reflectance up to 93%, by putting a perfect polished surface on it. Unfortunately, on contact with normal air, it rapidly degrades from this level immediately.
This is why they do it in an inert atomsphere, and do a special protective oxide overcoat. Most of the oxide overcoats will degrade the reflectivity. Some special oxide overcoats will actually improve the reflectivity. The coatings are typically called things like Enhanced Protected Aluminum (there is an even better one done in silver, called Enhanced Protected Silver. I use it in some products, the Enhanced Protected Silver coating costs roughly 1 dollar per 144 sq. in. that is coated).
Nickel makes for a rather bad reflector surface, at 72% reflectivity, though it does not degrade much on exposure to air, though it is commonly used for reflectors.
http://environmentalchemistry.com/yogi/periodic/Ni.html
Though you can get a rather specialized hi phosphorous Nickel that is rumored up to 80% reflective if it is done properly.
Also if looking this stuff up, note that solar reflectance, and normal reflectivity are two different numbers, and even the folks that use the terms get them mixed up. Solar reflectance includes IR and UV reflectance, though some only use visible + IR to make the numbers look better. In the end, it is always best to ask for a graph of reflectivity vs. wavelength. IMHO, the areas that are best considered are from 400nm to 700nm which is roughly the visible spectrum- even here, experts use different numbers on the ends, some making the range shorter, and some making it wider. On the ends, when you are looking at lumens, it is of little importance, as the human eye has very little sensitivity on either end.
Add all this up, and thats where flashlight manufacturer's claims that use the light source lumens for the flashlight lumens are easily 30-50% too high. Very few actually measure it. Where they are going wrong, is they are not including the losses of the reflector or the lens used.
Of course, nothing is free. I know 10 sq. in. of OCLI HEA 2 sided coated AR glass goes for 6 dollars cut. It sure is nice to gain an extra 6-8% light though.
I use AR glass on a routine basis...
As of June 30, 2004, JDS Uniphase had approximately 6,000 employees.
The OCLI subsidiary currently employs approximately 1,600 people worldwide
"Throughout its 52-year history, JDS Uniphase (aka OCLI's mother company) has provided its customers with high-quality optical products utilizing multi-layer interference coatings. Our first products were anti-reflective filters for high-altitude aircraft cockpit instruments. Since the Company's inception shortly after World War II, we have developed sophisticated process techniques for manufacturing optical thin film coatings that enhance or modify the behavior of light."
http://www.ocli.com/pdf_files/products/hea_ds.pdf
(link is now broken)
Use this for the web page now:
http://www.jdsu.com/index.cfm?productid=481&pagepath=Products/Custom_Optics/Products/Antireflection_Coatings&id=4836
Direct link to PDF:
http://products.jdsu.com/assets/public/pdf/hea_ds_co_ae_021706.pdf
"Optical Coating Laboratory, Inc. (OCLI) is the world's leading manufacturer of high efficiency anti-reflection coatings for Display Applications. Leveraging over 50 years of thin film coating expertise... You won't find a better anti-reflection display enhancement product than OCLI HEAŽ anywhere in the world."
See more on the Luxar brand coating, http://www.abrisa.com/downloads/luxar.pdf and http://www.abrisa.com/Guide/GlareRe...inFilm_desc.asp
You can learn much more about AR coatings and glass, here:
http://www.ocli.com/pdf_files/produ...on_coatings.pdf
(this link is also now broken)
This very useful manual on AR coatings I've put up on my website here:
http://www.molalla.net/~leeper/ar_coa~1.pdf
Bleh, lexan windows (polycarbonate), numberous reasons why I don't like those. Personally if I have to go with plastic, I personally prefer 2 sided AR coated acrylic instead, with an abrasion resistant hard coat, if plastic lenses are really needed-but each has it's advantages and disadvantages.
A graph from one type of AR coating (refer to the OCLI manual I linked earlier, much more information, and better coating:
There was some confusion between soda lime float and water white glasses, so I'll clarify here, in case I confused folks:
Clearview is a very specialized soda lime glass (unique)(another variant is called optiwhite), that has reduced iron which is what gives the glass the characteristic green tint. The drop in the red and blue of clearview or optiwhite glass wouldn't be seen in the beam. (the quality of the AR coating can also effect this, and lower quality Magnesium Fluoride AR coatings will often have a blue or reddish tint, and Silicon Dioxide AR coatings-more durable but don't work quite as well, will have a cast that is other than green. Good quality Magnesium Fluoride AR coatings will have a very deep purplish look in the reflection, and Silicon Dioxide AR coatings will have a greenish cast.)
I know the dip in the red and blue is a combination of both the glass and the AR coating. You can see it in the non AR coated clearview. I also know that with even clearer glasses, the AR does cause a slight drop is red and blue.
Clearview is not as tough as the borofloat/borosilicate group of glass, and one of the leading materials that folks are familiar with from the borofloat/borosilicate group is Pyrex. It's toughness is the main reason it is used for labware. It's not as brittle, and it is more chemical resistant than soda lime glass, as well as handles thermal stresses better. This type of glass is also utilized for more critical applications, such as LCD displays.
Looking at OCLI's chart for HEA or HEA-2 (BBAR), you can see some drop in the red and blue in the AR coating itself. This is why you see a slight purplish hue in a direct reflection, this is the MgF2 based coating. A less common type is SIO2, which has a green hue in a direct reflection. The quality of the MgF2 coatings, and how well they are applied, when not done correctly, will show more of a red or more of a blue cast.
AR coatings can be applied by vapor vaccuum deposit, sputtering, and dipping. There is a big difference in the durability of the coatings, dipping being the worst of the bunch, but by far the cheapest.
One version of the higher end Borosilicate glasses for LCDs is made by Corning and is called Eagle 2000. Note the flat optical transmission curve.
LCD Borosilicate Glass
"The manufacture of float glass is a modern technique that has increasingly replaced sheet and plate glass production since the 1960's. It produces glass that is nearly as high in quality as plate glass but does so in a single operation which, like sheet glass, requires no further processing. The technique was pioneered by Pilkington Brothers Ltd. - molten glass is fed into a float bath of molten tin. The glass floats on the molten tin as a ribbon and cools gradually as it travels across the surface."
On a side note, here is the datasheet for a very common GE Lexan sheet, which shows a 85% transmission, or in other words, a 15% loss(for a point of reference):
http://www.molalla.net/~leeper/lexan.pdf
Since the link to the borosilicate glass datasheet was broken, I'll add it here:
http://www.molalla.net/~leeper/borosil.pdf
Example of transmission loss:
http://www.uqgoptics.com/materials_...CoatedGlass.asp
A comparison plot of 2 sided AR vs. Mineral Glass:
This was further borne out, when the lens of a HDS LED flashlight was replaced with mineral glass, during Peter Gransee's calibrated integrating sphere tests. It was found that the mineral glass alone caused an additional 13% loss (or was it 14%).
Above are some of the typical losses of various lens materials.
Useful reflector coating links (stuff which isn't found in 99% of flashlights, due to the high profit margins in the market):
http://www.mellesgriot.com/products/optics/oc_5_1.htm
Reflector coating comparisions, pass your mouse pointer over the coating on the side, it will then plot it. Most flashlights have the Nickel or Rhodium coatings on the reflector:
Optiforms Reflector Coating Comparision
Reflector coating techique comparisions:
Vergason Technology
A reference for one brand of reflector coatings:
Melles-Griot reflector coatings
More reflector coating reference material:
Lambda Research
AR Lens coating comparision, spectral graphs, etc:
Opti-Forms
A reference for Bright Nickel coatings, placing it at 55% reflectance (very common flashlight reflector coating):
Bright Nickel
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