Frosted Glass VS Scotch Tape

Okay, this is one of the neatest tricks – frosted glass + Scotch tape = clear glass.  You have to see this:

Is this magic?  Is this some kind of interstellar phenomenon?  It is certainly cool and mysterious, but it makes sense if you think about the way that light, reflection, and refraction work together.  This is a very simple analogy, but imagine a stretch of blacktop on a highway – when the sun is shining on it on a dry day, the blacktop is rough and not reflective at all.  The matte surface of the blacktop, if anything, has a diffuse surface that takes a beam of light and turns it into many beams of light, all separate and at less strength than the original – just like velour or a busted piece of porcelain.  I made a few images to express this phenomenon – the first shows a beam of light (incident light) hitting the surface of a matte object:

matte reflection

In the case of a frosted piece of glass, some light makes it through the glass (is refracted), but nothing in its original beam form – it is very diffused and spread, like so:

matte refraction

The Scotch tape trick is pretty cool, as it basically fills in the rough surface of the matte glass with the adhesive and whatever other goodies are in the glue on a piece of Scotch tape, allowing the light to pass through the frosted glass without a lot of extra refraction and diffusion.  The tape acts as a filler, in other words, giving less spread to the frosted glass, as below:

low refraction

Now obviously in the image above I have not accounted for real refraction, the normal angle of the glass, or any of the factors that would basically make the smarty pantses of the world say “a beam of light would never just pass through the two materials without SOME kind of refraction and diffusion!”  Well, you’re right.  My illustration was more to show that the diffusion would be less.  But does the overall point make sense?

Lighting 101 Posts

I’m getting a pretty warm reception to this little series I have been doing, so I am going to be throwing these “Lighting 101” posts in here from time to time.  I hope that people are enjoying them, and if you have suggestions – please email me or leave me a comment.

My basic idea for these posts is to provide a place for some introductory lighting knowledge, terms, and concepts.  I have a lot of faith and respect for the Lighting industry, and I want everyone to at least have a basic understanding of one of the most important things in everyone’s life.

You can search these posts by looking for the tag ‘Lighting 101’ in the search box, or by clicking the ‘Lighting 101’ category.

Lighting 101: HID Lamps

Have you ever been out shopping for replacement lamps for your home and saw acronyms like HID, HPS, MSR, and HMI?  These acronyms refer to lamps that are different than your typical household incandescent with a filament – HID stands for High Intensity Discharge lamp, and refers to lamps that utilize an electric arc to create light.  HID sources are typically very, very bright, and have a very high color temperature on average.  Color rendering indexes, and correlated color temperatures are all usually very high in HID sources.

High intensity discharge lamps don’t have a filament, and do not function like an incandescent lamp except for in the fact that they require electricity.  As a matter of fact, they’re about twice as efficient than incandescent Tungsten Halogen filamented sources, typically.  Once the lamp is ignited, it burns consistently and clean during its arc phase.

Take a look at this image:

HID sources usually consist of a few basic parts – an arc tube, arc electrodes, a metal salt, and a gas, usually of the halogen or chalcogen family on the periodic table.  Once the arc is ignited, the gas and metal salts heat and evaporate to a plasma phase, which greatly increases the light output of the lamp, and also makes it use less electricity.  HID lamps need a ballast to both start and maintain the arc that occurs in the arc chamber (the glass envelope part of the lamp) – to start, the ballast sends a high voltage across the arc gap, which refers to the distance between the electrodes.  Depending on the lamp, this could be as little as 500V or as much as 3500V.  Once the arc is established, the ballast drops the voltage down to a “maintenance” voltage, maintaining the arc and keeping the lamp lit.

HID laps are similar to welding – they put off high amounts of very high color temperature light that can hurt your eyes if you look directly at them.  HID lamps are sort-of like controlled welding in a way, except the deposit that welders make is not quite how HID lamps operate.  HID lamps are made of all sorts of chemistries, from Mercury Vapor lamps (the streetlights that are extremely white/blue), Sodium Vapor lamps (very yellow/amerish light), to Medium-Source Rare-Earth or Hydrargyrum Medium-Arc Iodide lamps.

What?  Hydrargyrum?  Did someone burp?  Hydrargyrum is another name for Mercury.

HID sources are used all over the place.  When a need for wide area lighting is required, HIDs do the trick – next time you’re at your favorite big-box retailer, look up and see what’s lighting the store.  Fluorescent tubes are HID lamps, and they are found everywhere.  HIDs are used in Film and TV Lighting, moving lights, and anywhere that a bright, consistent and efficient source of light is needed.  They’re everywhere – the World Trade Center site, atop the Luxor in Vegas, in some car headlamps, video projectors, and in millions of other places.

Handling of HID sources takes some extra care; when changing out an HID source, the best bet is to use some sort of face shield and eye protection while it is still warm.  As the lamp cools down, the lamp is even more fragile than in its cooled state, and could possibly explode in your face if you weren’t careful.  Never look at the source, as mentioned before – this can cause overall blindness and loss of night vision.  HID sources are usually high voltage sources too when in operation.  Take extra care!

Lighting 101: Luminance VS Illuminance

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Do you know the difference between “luminance” and “Illuminance?”

When talking with students, or people who just love light but don’t know much about it, there are a few terms that are used sometimes incorrectly – “That table has a high illuminance value,” or “you can measure the luminance coming from that light source.”  Not quite right.  Do you know how to use these integral lighting terms?

It’s easiest to remember if you think of it in terms of what is exactly happening when a beam of light strikes a surface.  Let’s look at this small image:

An incidence of light striking a surface.

Incident light.

In this example, we are looking at one beam of light hitting a surface.  The beam of light is red, and the surface, obviously is green.  The red arrow represents the incident light, or the beam(s) of light striking the surface.  Another time I’ll write about the Law of Reflection and what it means, but also look at this diagram of what happens when a beam of light is reflected:

Incident light and reflected light

Incident Ray vs. Reflected Ray

Figuring out the difference between luminance and IL-luminance is as simple as remembering what has happened to the beam of light; the incident of light hitting a surface – a beam of light that is travelling towards a surface – is what is measured when talking about illuminance.  Illuminance is measured as the amount of light striking a surface.  The incident light.  If we had a light meter and wanted to measure the amount of light that was striking the surface we were measuring, we would hold the little white bubble towards the device shooting the light.  These meters are also called “incident meters” for a reason.

On the other side of this, luminance, is what we measure off of the surface that has light hitting it.  Luminance is the measurement of the product of the incident light and the surface – anything that is reflected.  Luminance is also considered the human perception of brightness, or how bright we perceive the light that is reflected off of the surface.

Make sense?  Think of it like this – IL-Luminance, IL, I = Incident Light.  Illuminance is measuring the incident light.  Luminance is what’s leaving the surface – L = leaving.  Illuminance is measuring incident, luminance is measuring what’s leaving.

I hope that helps!