Questions about Brightness

[michaelfox]

Hello all

 

I am a student at university and I have been doing some research on lighting.

I am somewhat confused about the concept of brightness. Specifically, on, what really affects brightness?

Amperage? Voltage? Both?

 

It makes sense that, more amps (less voltage) would provide more brightness.

However, I think I have seen examples where lights with less power and voltage are still brighter than

lamps with bigger wattage (acls, for example?)

 

Could you shed some light over these doubts?

 

Thank you

 

Michael

 

 

[Shez]

You should investigate power. That's the product of voltage and current and is a better measure for your purposes than either of them.

Also consider efficiency and beamwidth.

[Robin D]

Where does one start?

 

It mainly depends on the source. Given a tungsten lamp, the wattage (voltage x Amps) will dictate brightness. However the optics of the luminaire will also have a significant effect.

 

Different light sources though generate different lights (at different parts of the visual spectrum) and that's been known for donkeys years. Hence a florescent lamp produces more light for less watts as a general rule. Modern house lamps are often CFL's that provide 100W tungsten equivalence for about 14W input ... hence low energy.

 

In this field there are so many different sources these days, and it seems every year or so a new one arrives. Many of these generate light more efficiently, but none of them will suit all tasks and many have serious downsides.

 

I'm a little puzzled that a university student appears to know so little about the subject, much of which should have been covered in school. What sort of course are you on?

[michaelfox]

You should investigate power. That's the product of voltage and current and is a better measure for your purposes than either of them.

Also consider efficiency and beamwidth.

 

Yes, but I am under the impression that two bulbs, with the same wattage, can have different brightness levels, if we, for example, lower the voltage on one of them.

 

Am I right?

 

Where does one start?

 

It mainly depends on the source. Given a tungsten lamp, the wattage (voltage x Amps) will dictate brightness. However the optics of the luminaire will also have a significant effect.

 

Different light sources though generate different lights (at different parts of the visual spectrum) and that's been known for donkeys years. Hence a florescent lamp produces more light for less watts as a general rule. Modern house lamps are often CFL's that provide 100W tungsten equivalence for about 14W input ... hence low energy.

 

In this field there are so many different sources these days, and it seems every year or so a new one arrives. Many of these generate light more efficiently, but none of them will suit all tasks and many have serious downsides.

 

I'm a little puzzled that a university student appears to know so little about the subject, much of which should have been covered in school. What sort of course are you on?

 

I did literature in school and am now doing some research for a possible arts project.

 

So, considering the relativist side of brightness, would something like the candela output be a more faithfull and reliable source of information?

[paulears]

An 'ARTS' project? It's unusual (and a very steep learning curve) to pick what is essentially physicsnas the basis of a university project when you don't quite have a handle on it when you kick off.

 

There is a direct correlation between the amps and volts and brightness - but you're missing the important point. In conventional lamps electricity is used to heat up a filament, which then produces light and heat. The other types of light source have very different systems for producing their output. So it doesn't really matter if we look at low voltage lamps or high voltage lamps. Over-running a tungsten lamp increases light output and colour temperature - so that's purely the product of higher voltage. Lower voltage needs more current, more current means heavier and thicker filaments - so there's plenty of data - but what is the purpose?

 

 

Two bulbs can obviously be different brightnesses if you lower the voltage - this is what stage people have been doing for years and years.

 

What impacts on brightness? Simply the design of the lamp. If you are doing an arts project, then surely you should be dealing with the quality of the light, the aesthetics properties of not just the lamp, but the lantern it's installed in. These are much more important for understanding what makes light 'good'. Are you sure you've not jumped the gun a little, and misunderstood what they actually want? If they do wish you to understand the physics, it's going to introduce a huge number of quite complex study areas. Arts courses tend to focus (pun intended) on what you can do with the light, after it's produced. It's a bit like somebody studying the chemical composition of Prussian Blue when they'd normally be painting with it!

[adam2]

Total light output of a lamp is determined by the type of lamp, the wattage, and the design.

Incandescent lamps are the least efficient type in general use, but remain popular as they are easily dimmed, start instantly, cheap to buy, have good colour rendering, and are available in a vast range of sizes.

Wattage is roughly proportional to light output, with larger sizes slightly more efficient.

For example a 1,000 watt halogen lamp will give slightly more than twice the light of a 500 watt one.

Incandescent lamps can be made brighter by making the filament hotter, but this reduces the lifetime.

For example a 12 volt 50 watt projector lamp gives more light than a 12 volt 50 watt industrial lamp. The projector lamp lasts 50 hours, the industrial version for several thousand hours.

Low voltage lamps are usually more efficient than mains voltage ones, for example a 12 volt 60 watt vehicle headlight bulb gives more light than a 240 volt 60 watt household bulb.

 

Discharge lamps are more efficient than incandescent and are widely used for street and industrial lighting, and some specialist theatre applications such as movers.

They are expensive, take a few minutes to warm up, and can not easily be dimmed, but save a lot of money on electricity.

 

Flourescent lamps are also very efficient and therefore in widespread use, cheapish to buy, long lasting, wide choice of colours, but only dimmable with diffeculty and therefore little used in theatre.

 

LEDs vary a lot and are relatively new technology, they are improving rapidly and are already a realistic alternative in many cases.

 

Finally remember that brightness and total light output are not the same thing at all !

A 100 watt household bulb gives dozens of times more light than a 1.5 watt torch bulb, yet the torch bulb in an effective reflector could well be brighter if viewd in the correct direction.

You can read by the light of 60 watt vehicle headlight at half a mile away, not a chance with a 100 watt bare bulb without a reflector.

[michaelfox]

Finally remember that brightness and total light output are not the same thing at all !

A 100 watt household bulb gives dozens of times more light than a 1.5 watt torch bulb, yet the torch bulb in an effective reflector could well be brighter if viewd in the correct direction.

You can read by the light of 60 watt vehicle headlight at half a mile away, not a chance with a 100 watt bare bulb without a reflector.

 

I see! So, in this case, It is not relevant to compare wattages at all. It is just the design (reflector) that makes the car headlight brighter than the bare bulb !?

So, I have read that a PAR 36 28v ACL is brighter than a 1K Par64 CP 60. Am I also to think that this is also due to the design of the lamps?

 

Thank you!

 

Michael

[pritch]

It is relevant to compare wattages, but that's not the only thing that makes a difference.

 

Let's stop getting tied up in theatre lighting for a second, and look at the humble GLS lamp - the normal domestic light bulb.

 

If I take two different wattage tungsten GLS lamps from the same manufacturer, and put them in a bog standard ceiling pendant with no reflector or shade, then the 100W lamp will be brighter than the 60W.

 

Something else you'll notice about a lamp in a pendant fitting is that it doesn't just light up what's below it, it'll also cast some light onto the ceiling. Now, if you add a reflector it won't make the lamp any brighter, it'll merely give you some control over where the light goes. The cheap conical type of lamp shade that you'll often find at home often has a white interior, which acts as a reflector - if you put one on a pendant then you end up with less light on the ceiling.

 

BUT... I've compared two similar lamps there. What if I take a lower wattage one, such as a 40W halogen GLS lamp? Despite using a lower power, it'll be about the same brightness, or brighter than the 60W normal (non-halogen) GLS.

 

Thinking about theatre lighting now, back in the bad old days when the Patt 45 was about, it didn't have a reflector - filaments emit light around a full 360degrees, so a lot of the light went into wastefully illuminating the interior of the lantern. Nowadays, I personally can't think of anything that doesn't have a reflector either in the lantern or as part of the lamp. This means that if I take two identical lamps, put one in a reflectorless lantern, and one with a reflector, I'll get more light out of the front of the lantern.

 

When you start to look at something like a fresnel or profile lantern, you can adjust the lens(es), which can give the impression of a brighter light - if I focus a lantern so that there's a really wide spread coming out of it, it'll appear dimmer. Focus it down to a tight spot, and it'll appear brighter. Do I have more light? No - I've got the same amount of light, but in a smaller area.

[Matthew Robinson]

When you start to look at something like a fresnel or profile lantern, you can adjust the lens(es), which can give the impression of a brighter light - if I focus a lantern so that there's a really wide spread coming out of it, it'll appear dimmer. Focus it down to a tight spot, and it'll appear brighter. Do I have more light? No - I've got the same amount of light, but in a smaller area.

 

Light intensity is inversely proportional to the illuminated area squared, IIRC from A-level physics.

 

For the sake of completeness, there are three main types of lamp:

Incandescent: a small piece of wire (the filament) is heated up until it glows and gives off light.

 

Discharge or fluorescent: a high voltage is present across the lamp (it's easier to think about this happening in a fluorescent tube) and electrons, which can be thought of as little spheres, crash into atoms. In the process, the electron loses kinetic energy to the atom, but an atomic electron can only have discrete amounts of energy at once, so it 'rejects' this energy. The way that atomic electrons reject energy is to give out a lump of light called a photon.

 

LED: In a semiconductor, there are some atoms which have too many electrons and some which have too few (this absence of electrons is called a hole- think of it as a bubble in a pint of beer), and the process of making an area of semiconductor have too many or too few electrons is called doping. As a voltage is put across the semiconductor, the electrons (which have a negative charge) are drawn towards the positive electrode and the holes (which have a positive charge) are drawn towards the negative electrode- a little bit like magnetism (opposites attract). As the two types of doped material (one with lots of extra electrons, and one with lots of holes) are kept in discrete blocks, the joint between them is called the depletion region. This is where holes and electrons recombine, giving off energy in the form of a photon.

 

HTH, even if it is probably slightly deeper than you wanted to go, but it does explain the differences between different types of light generation.

[paulears]

The problem here seems to be we're veering towards a BSc, rather than an Arts qualification, where to be honest people are more interested in exactly the right shade of lavender, or how yellow the straw actually is. With the greatest respect to Michael, his posts indicate that A Level physics is a bit of a jump from double-science GCSE or whatever. I might be doing him an injustice - but I'd really question if physics is the right area of study.

 

Michael - what's the purpose of the idea - you've kind of set yourself something a bit tricky - because you've got to master the science and then apply the maths. Photometric data - essential to get meaningful results, yet when you try to collect it, you get all sorts of different units - Candellas, Lumens, spectra, wavelengths, and loads of other units that you'll have to attempt to quantify in some contextual way - not beginners stuff. If you then start to add optics, as in how the fixtures collect and control the light - the maths gets quite deep.

 

Now that LED seems to be a more and more common light source, lots of people are trying to get meaningful data on how they perform, but it's very difficult to find common ground. You'll find loads of people asking about the brightness of specific fixture compared with others, and nobody ever seems to come up with absolutes. Is a single candle in a cathedral at midnight bright? How about the difference turning on a 4000W Arri discharge Fresnel, outside on a sunny day - will you actually be able to see it?

 

There's brightness as shown on a meter, and brightness as perceived by the eye.

[Matthew Robinson]

The problem here seems to be we're veering towards a BSc, rather than an Arts qualification

 

I decided to give an explanation of how lamps work because higher in the thread there were a lot of people discussing the different types of lamp, but no explanation as to the actual differences. I tried to simplify it as much as possible! :P

[Ken Coker]

Michael

 

I'm lighting a gig in central Manchester tomorrow.

I can probably give you half an hour to show you a tungsten, an LED and a discharge source. I can probably answer a few questions too.

PM me.

 

 

 

KC

[Robin D]

Some really useful input here for the OP.

 

One point of clarification as Matthew's, Pritch's and my contributions may not make sense together. An incandescent lamp is the same sort of lamp that Pritch spoke about with GLS and I said was a tungsten. The piece of missing information is that the material that the filament is made from, and the gas within which it burns (within the glass envelope) both affect the brightness and colour. Hence if you look carefully at a 'halogen' GLS bulb, you will see that there is a small bulb within the bulb. The smaller one is made of quartz to stop it melting but contains one of the halogen gases. A standard bulb is filled with an inert gas.

 

As Paul and other have rightly pointed out, this whole are can get extremely complex and is more science than art.

 

Great offer from Ken.

[Junior8]

There's brightness as shown on a meter, and brightness as perceived by the eye.

 

As so often Paul has got to the nub. Brightness is about perception and (unless you are dealing with photometry in a perfect blackout) is all about relativity. Just think how distracting one tiny exit box can be sometimes even if viewed through 10kw worth of white light. (If you look up at a modern rig for any length of time all you can see in the end is the bl**dy leds twinkling away.)

 

That's why for all the specifications and throw and spread charts and different sources you don't really know what it's going to look like until you've rigged it and switched on - and the swearing starts.

[Seano]

The piece of missing information is that the material that the filament is made from, and the gas within which it burns (within the glass envelope) both affect the brightness and colour.

Not directly. Those design details affect the temperature that the filament can [be designed to] run at.

The temperature alone determines the colour (hence "colour temperature", quantified in Kelvin, being used to describe the colour of the light). Possibly a bit too much physics for the OP, but "Black-body radiation" is the thing.

 

Great offer from Ken.

Yup. Definitely one the OP should take up if he can. :)