What's the point of these fins in flourescent lightbulbs?

[Nantes]

Many linear flourescent lightbulb fixtures that I see have this series of transversal fins, such as in this picture: http://www.bizrice.com/upload/20130409/T5_14w_embeded_linear_fluorescent_louver_light.jpg. What are they for? They are certainly not for efficiency, as the lamp would be more illuminating without them. My best guess is that they mitigate glare if you look at them from certain distant angles. Anybody can think of another reason?

 

[russ_watters]

They are probably there for either pure aesthetics or to cut off the view angle a bit.

 

[duhuhu]

They are used to help disperse the light in a softer gradient across a larger area.

When you have a plain light bulb, it creates a circular light gradient with the brightest point in the middle, but if you don't want to have that point be much brighter than the rest of the circle, you cut out some of the light and send it elsewhere. The same theory applies for these light fixtures.

You will note that in hallways or large rooms that have these lights, there is often 10 feet between light fixtures. they don't want the light to just go straight down to whoever is directly below the fixture, so using these reflective "fins", they can redirect a portion of the light outward to produce a softer light gradient over a larger area so that they can have these lights farther apart and have greater effective efficiency.

Source: My sister, A Lighting Engineer for Bega Lighting Corperation

 

[Nantes]

Thank you for the awesome reply and asking your sister about it. Still, it makes no sense to me. Let me illustrate it with a drawing.

If my logic is correct, the purple and violet rays coming from the left side and reflected on the fin would have perfect equivalents on the right side, that would have "filled in" for the reflected ray, if the fin was not there. Any rays on the right side and being reflected would also have other rays filling in for them that came from the left. All the fins seem to do is provide opportunity for light to be lost, reflected inside the fins, as per the orange ray. Fins only make sense if they are placed on the corners, where there are no other side of the lamp to provide "fill in" rays.
Can you show this to your sister and see what she thinks? Thank you!

 

[duhuhu]

So the way she explained it was that because it captures some of the rays (orange) that would go straight down(ish), that helps make the middle a little dimmer, and the purple rays get bounced out farther away from the fixture, and the dotted purple lines end up going close to straight down. that way you can have a fairly even distribution of light in a decent radius from the fixture and more light being dispersed outward from the light which is dimmer, but due to the other light fixtures around it producing the same effect, it can produce an even light distribution across a large area instead of just a bunch of bright spots surrounded by dark areas.

If you are in a building with a large room, then even when the lights are turned off, one or two will stay on. go check out what the dispersion pattern is under one of those lights, and then think about how that will fit together with all the other lights in the room. Then if the fixture is on hinges (like most of them are) make it swing down and then look at it again. it will be a lot brighter in the middle with less light along the edges of the pattern.

She used some really odd words that I have never heard before, but that is what I got out of the conversation I had with her... But she suggested looking at the fire alarm flasher which, if you have a newer model, has all kinds of crazy angled mirrors in it, but it disperses the light in a way such that anyone in the room can see it clearly.

The engineering that goes into light fixtures is crazy, they can create models of exactly what the light will do and how it will disperse in a room. or more so how it will not. You will notice that there are tons of different types of fixtures for those fluorescent bulbs. each one is useful for a specific room with a specific pattern to the lights that are used with specific lighting needs. each one has different dispersive properties depending on what the room is to be used for. Just walking from my office to the restroom, I pass under 7 different types of fixtures for the same exact bulb.

 

[sgb27]

just to add though, a large proportion of the light that is "caught" by the fins will be reflected back up into the unit (it will be much nearer to vertical than the orange line in your diagram) and some of it will come out again at a wider angle, so that keeps the efficiency/overall brightness up.

A very similar concept (only using refraction rather than reflection) is used in all LCD TVs to make the brightness more uniform at wider viewing angles. In fact some displays used in the centre console of cars are deliberately designed so that the driver and passenger get to see a much brighter image that someone looking head-on.

 

[Nantes]

just to add though, a large proportion of the light that is "caught" by the fins will be reflected back up into the unit (it will be much nearer to vertical than the orange line in your diagram) and some of it will come out again at a wider angle, so that keeps the efficiency/overall brightness up.

I don't think the light comes back out of it after so many reflections, and if it does it will likely come out so weakened that it will hardly help with illumination. See the drawing in the link below.

So the way she explained it was that because it captures some of the rays (orange) that would go straight down(ish), that helps make the middle a little dimmer,

But then you have this fixture maker bragging of their patented fin design that makes the top closed, so that less light is captured there, making the whole thing more efficient. They also mention that their fins are much thinner than usual, further helping with avoiding light loss. The page is in Portuguese, I could translate if you wish. http://www.lumicenteriluminacao.com.br/pt/tecnologia/lumiotic.html

and the purple rays get bounced out farther away from the fixture, and the dotted purple lines end up going close to straight down.

That still makes no sense to me. Rays from the dotted lines would go exactly where the reflected ones would go, provided there was no fin there blocking them. So reflected rays are no more, no less dispersed than dotted ones would be!

that way you can have a fairly even distribution of light in a decent radius from the fixture and more light being dispersed outward from the light which is dimmer, but due to the other light fixtures around it producing the same effect, it can produce an even light distribution across a large area instead of just a bunch of bright spots surrounded by dark areas.

I understand the idea and desired end result, just not how the fins actually help you get there. By the logic above, the same result would be achieved without them.

Just walking from my office to the restroom, I pass under 7 different types of fixtures for the same exact bulb.

Let's consider that that variety could also be due to the fixtures having been replaced over time and the manufacturer no longer produces that model, so they had to make do with another. Or the office decided to save money and buy cheaper fixtures. Or many other reasons :P

 

[duhuhu]

I am just telling you what she told me. I do know that you have to follow the rays on your diagram all the way to the floor in order for it to make any sense, and you have to do every direction from every point on the bulb that produces light, your 5 ray diagram does not do my argument ANY justice, but if you were to do an actual analysis, what I have given you holds water.

Just remember to follow the rays using the laws of reflection, and it will go to where it is going to go. If the fins on the fixture did nothing, then offices would just have light bulbs in the ceiling with no fixture to disperse their light like what they had back when these bulbs were first invented. I apologize for not being able to explain it in a way that makes sense to you, maybe you could email the company and ask them to tell you what its all about.

 

[Nantes]

I'm sorry, I didn't mean to be rude in any way! I'm straightforward with what I don't understand, and in no way I mean to be ungrateful for your excellent help so far.

I know they have a point, or the manufacturers wouldn't waste extra resources in manufacturing them. I found the following text in a document from the Light Studies Lab of the faculty of Architecture of a Brazilian University, which translating reads:

"Aleta (fin)
A small tab, fin or grill. Placed in series so as to impede the direct vision of the lightbulb from a range of angles. It serves as an anti-obfuscating resource, and contributes to the distribution of the light flux, together with the reflector, in more modern lamps".

So this confirms both your sister's explanation, as well as my initial theory that they serve to stop you from looking directly at the lamp from far away. But for fully understanding your sister's point I think I would need a tridimensional diagram. Does she recall seeing something like this in one of her college textbooks? I can try to find it.

Once again thanks for the great work so far, and sorry for any misunderstandings.

 

[duhuhu]

for fully understanding your sister's point I think I would need a tridimensional diagram.

That is very true, Everything she did was in a special CAD program and it took hours to run each time that she needed to see what the dispersion would look like. I will ask her, but she took 12(ish) different courses that directly related to lighting, so I doubt that she will be able to remember which textbooks dealt with what aspect of lighting. I can also try to find one of the university's websites that might have it for you.

Sorry about the misunderstanding, it was not my intention to be at all hostile.

 

[Rabbit]

I believe that the primary purpose is to obfuscate view of the direct light source. (I'm not sure the manufacturer is correct is his use of the term 'anti-obfuscating' or, at least, it's confusing.) Certainly, it would reduce the overall light output, as no reflector is perfectly efficient. It's a lamp shade, in other words, used for much the same reason we use lamp shades around incandescent light bulbs. We do not fit those to increase the efficiency of the lamp, but to reduce the harshness of shadows produced by the pin-point source and shield our eyes from direct view of it.

If the light source can be viewed directly our eyes adjust to the bright source, making the apparent brightness of its surroundings less. Therefore shielding the source will improve the perceived brightness of the illuminated area to our eyes. (Pet peeve of mine: poorly designed streetlights that make it difficult to see because they are so bright!)

The shape and reflectiveness of the shield is a secondary consideration, in my opinion. Given that having a shield at all is more desirable than none, efforts are then made to use it to even out the light distribution and make it as efficient (or less inefficient) as possible by reflecting the light that would otherwise be absorbed by a non-reflecting shield.

 

[Nantes]

Very interesting insight you bring in, Rabbit. I had never realized that too bright a light could make the environment look less bright! That indeed makes sense, given our adaptive vision.

On the other hand, one rarely ever looks directly up to the light source, so an unshielded lightbulb would only be present on the field of vision peripherally. Even then, the adaptive vision effect still kicks in?

"anti-obfuscating" was a wrong translation, it should have been "anti-obfuscation". By "obfuscation" they mean "the difficulty/discomfort in vision caused by an unfavourable distribution of luminances, with intense brightnesses or excessive contrasts. Obfuscation can be direct, by means of a light directed to the visual field, or indirect, by means of reflective, bright or transparent surfaces, if one considers a luminance above 200 Cd/m² as being unpleasant".
Bear in mind this is originally in Portuguese, so the technical terminology used may deviate from the English one. Or I suck at translating :P