Reef Central Online Community - View Single Post - Alright Newbie; Let's shed some light on this subject.

WaterKeeper · 03/18/2005, 12:40 PM

Didn't get much sleep again last night; did you Newbie? Too busy calculating the number of Lamberts in a differential solid angle's total steradians I bet.

I closed yesterday with a list of lighting terms. Most of those I hope you never need to use but some of those are pretty common and probably need explanation.

Go to the good old Home Depot's lighting department and you see that most lights have the total lumen output shown on the box. Gosh darn? you say, That Sylvania Cool White Deluxe certainly packs in the lumens.?

Lumens are not all that hard to understand. Just take a look at that muscle car sitting out in the driveway. It has a 3.5 liter 351 horsepower engine. Lumens are much like horsepower. They give a general idea of how much power a light source packs. In fact, lumens can be directly compared to an electrical standard, the watt. One watt equals 683 lumens give or take a few. So a 100 watt incandescent gives out as many lumens as a 100 watt fluorescent of a 100 watt metal halide, Right?

In theory they would all be equal giving out 68300 lumens each. However, let's look again at that muscle car. Sure it has 351 horsepower but how does it compare to other cars that also have 351 Hp engines? We all know that things like torque, gear ratio, vehicle weight, and tire type will affect how much of that engine horsepower will translate to speed in the quarter mile. That's much like lumens. They tell you how much light is theoretically available but don't say a thing about how much of that light is usable or will provide the desired result in your tank. None of the lights listed will actually produce 68300 lumens. It is always much less. Some of it is lost in heat or absorbed by the glass envelope of the light itself. There are many variables involved.

OK, it is time for a practical exercise here. We could do this with some heavy duty math but a little visual demonstration is far simpler.

Take that trusty old Maglite and adjust it to provide an even beam. Hold it about a foot away from the wall and note the size. Now move that light to two feet away and now look at the size of the beam. It sure has grown hasn't it? In fact, it now covers an area four times as large as it was at one foot. Now move it back to 4 feet. The illuminated area is now 16 times as large and, you will notice, much dimmer. It dims because the bulb in the Maglite only gives off a fixed number of lumens. As the light is moved further from the wall the lumens must cover a larger and larger area with the same amount troopers (lumens). Eventually, as the light is moved further away, the troopers are spread so thin they are barely visible. The intensity of light falling on a given area is defined in lux. Lux have the unit of lumens/ meter². As a light source moves further and further from what it illuminates the lux decrease as the inverse of the square of the distance. In other word doubling the distance doesn't halve the light intensity but decrease it to one-forth as much.

Another interesting thing is happening in our demo that we should note. What are we seeing when we look at the light beam on the wall? We are not seeing the light source itself but rather a reflected image of the actual source. That image is, in most cases, not identical to the light source. For one the image is getting more radiation than our eye. Why? Because the light needs to travel the distance from the wall, where the image is formed, to our eye. This trip from the wall to eye further reduces the lumens we see. One other thing before you turn the Maglite off. Instead of holding it perpendicular to the wall move it so it hits the wall at a angle. The beam appears as an egg shaped ellipse with the most lumens hitting the small end of the egg and decreasing as we move toward the large end. Another thing is happening, depending on the surface, some of the light hitting the wall may be absorbed or it may shift its spectral output from that of the original source. In other words a color shift.

We have two separate factors when we are dealing with light. Actual emission, coming from the source and reflected emission coming from the area where the light strikes.

Actual emission is important in what the critters in our tank's "see" from the lights we buy. Reflected emission has several effects. It allows us to add reflectors into the hood to direct light to the area we wish to illuminate and it effects how our eyes see the objects under illumination. Both of these are important considerations in choosing our lights.

Well it's Friday and I'm sure most of you want to knock off early and suck down a few near beers over at the PX. We'll cover more next time.

03/18/2005, 12:40 PM	#16
WaterKeeper Bogus Information Expert Join Date: Oct 2000 Location: SW Ohio Posts: 16,147	Didn't get much sleep again last night; did you Newbie? Too busy calculating the number of Lamberts in a differential solid angle's total steradians I bet. I closed yesterday with a list of lighting terms. Most of those I hope you never need to use but some of those are pretty common and probably need explanation. Go to the good old Home Depot's lighting department and you see that most lights have the total lumen output shown on the box. Gosh darn? you say, That Sylvania Cool White Deluxe certainly packs in the lumens.? Lumens are not all that hard to understand. Just take a look at that muscle car sitting out in the driveway. It has a 3.5 liter 351 horsepower engine. Lumens are much like horsepower. They give a general idea of how much power a light source packs. In fact, lumens can be directly compared to an electrical standard, the watt. One watt equals 683 lumens give or take a few. So a 100 watt incandescent gives out as many lumens as a 100 watt fluorescent of a 100 watt metal halide, Right? In theory they would all be equal giving out 68300 lumens each. However, let's look again at that muscle car. Sure it has 351 horsepower but how does it compare to other cars that also have 351 Hp engines? We all know that things like torque, gear ratio, vehicle weight, and tire type will affect how much of that engine horsepower will translate to speed in the quarter mile. That's much like lumens. They tell you how much light is theoretically available but don't say a thing about how much of that light is usable or will provide the desired result in your tank. None of the lights listed will actually produce 68300 lumens. It is always much less. Some of it is lost in heat or absorbed by the glass envelope of the light itself. There are many variables involved. OK, it is time for a practical exercise here. We could do this with some heavy duty math but a little visual demonstration is far simpler. Take that trusty old Maglite and adjust it to provide an even beam. Hold it about a foot away from the wall and note the size. Now move that light to two feet away and now look at the size of the beam. It sure has grown hasn't it? In fact, it now covers an area four times as large as it was at one foot. Now move it back to 4 feet. The illuminated area is now 16 times as large and, you will notice, much dimmer. It dims because the bulb in the Maglite only gives off a fixed number of lumens. As the light is moved further from the wall the lumens must cover a larger and larger area with the same amount troopers (lumens). Eventually, as the light is moved further away, the troopers are spread so thin they are barely visible. The intensity of light falling on a given area is defined in lux. Lux have the unit of lumens/ meter². As a light source moves further and further from what it illuminates the lux decrease as the inverse of the square of the distance. In other word doubling the distance doesn't halve the light intensity but decrease it to one-forth as much. Another interesting thing is happening in our demo that we should note. What are we seeing when we look at the light beam on the wall? We are not seeing the light source itself but rather a reflected image of the actual source. That image is, in most cases, not identical to the light source. For one the image is getting more radiation than our eye. Why? Because the light needs to travel the distance from the wall, where the image is formed, to our eye. This trip from the wall to eye further reduces the lumens we see. One other thing before you turn the Maglite off. Instead of holding it perpendicular to the wall move it so it hits the wall at a angle. The beam appears as an egg shaped ellipse with the most lumens hitting the small end of the egg and decreasing as we move toward the large end. Another thing is happening, depending on the surface, some of the light hitting the wall may be absorbed or it may shift its spectral output from that of the original source. In other words a color shift. We have two separate factors when we are dealing with light. Actual emission, coming from the source and reflected emission coming from the area where the light strikes. Actual emission is important in what the critters in our tank's "see" from the lights we buy. Reflected emission has several effects. It allows us to add reflectors into the hood to direct light to the area we wish to illuminate and it effects how our eyes see the objects under illumination. Both of these are important considerations in choosing our lights. Well it's Friday and I'm sure most of you want to knock off early and suck down a few near beers over at the PX. We'll cover more next time. __________________ "Leading the information hungry reefer down the road to starvation" Tom Current Tank Info: 130 Now out of service and a 29 Last edited by WaterKeeper; 11/22/2005 at 12:49 PM.