Light is great, it lets us find our sandwiches and saves our shins from being mauled by feral coffee tables and means we don’t all have to learn braille.
But there’s different ways to achieve illumination in our daily lives and different types produce different shadows.
If you’re painting a picture with shadows you might want to know what sort of shadows you should be using – Not all lights produce the same sorts of shadows.
A ray of golden…
In this tutorial I’ll be talking about rays a lot. this is for my convenience and possibly yours too – Rays aren’t real things. There isn’t a line connecting a light to whatever it’s illuminating.
It’d be more accurate to say that a ‘ray’ is the path of a photon between the illumination source and it’s destination – And let’s ignore things like refraction and diffusion caused by atmosphere and dust. For this tutorial, ‘Ray’ just means the straight line between a source of illumination and it’s destination.
A point light is a light that emanates from a single point like a light-bulb or a candle. These aren’t actually real ‘points’ but they’re small enough that they can be treated as basically a little dot that radiates light in a sphere around itself.
In the above example, the light is a normal environment destroying incandescent bulb, or a normal environment destroying CF bulb. the light radiates in all directions – horizontally, vertically, 45 degrees and all points between. I’ve drawn in some lines to show this for convenience.
The angle of the light on the right side of the table is different from the left.
The light hits the table, which is opaque and bounces off – in other words it doesn’t go through - Since this is a very simple diagram, I haven’t shown any of the light bouncing around either. The result is that there’s an area where the light’s not reaching, which as you already now, is the shadow.
For literally dozens of years, people have been using natural daylight to see by, in an effort to reduce their electricity (Or in some cases, gas) bills.
The sun is a giant sphere 864,300 miles wide, made of luminous plasma. It is not a point source. It’s a giant glowing ball. Luckily for us we’re generally about 8 light minutes and a few seconds away form it so by the time all that light’s reached us it’s diffused out.
The rays reaching us are not parallel, but because we’re so far away and the sun is so large, the angle of difference between any two rays hitting our table is so small it’s easier just to assume they’re parallel.
Therefore the angle of the light passing on the left side of the table is the same as that on the right.
True parallel lighting is almost impossible to find. Even lasers spread over distance.
Area lights are tricky. Technically speaking, all light sources are area lights – Some are jsut small enough to treat as points.
For this tutorial I’m going to split the topic in two, to deal with real-world situations.
The most common area light you’ll encounter that’s not the Sun, is going to be fluorescent tubes. These are basically glowing cylinders for the purpose of this tutorial, so I’ll call them Tubes. I’ve drawn them end on for convenience, so you’re looking a circular cross section.
The first thing to notice is that because there’s three lights right next to each other, they’re both brighter than the previous image, and they cast over-lapping shadows.
There are three shadows being cast, each one being partially lit by the other two tubes.
Generally speaking this is not a nice thing to have to work with, so in most places that use them a dffusing panel is placed over the tubes to average out the light form the tubes.
This gives a nice bright light with soft, diffuse or ‘feathered’ shadows. This is because the rays of light coming form the tubes have been altered by the diffusor to fill in the shadows, giving the impression of soft gradation.
You can see that at the edge of the table where the shadow starts it’s fairly crisp and firm edged, but the further away from the casting edge it gets, the softer the shadow becomes.
This happens when the sun shines through clouds (Which are made of water vapour that’s condensed into billions of miniscule water droplets which are acting as lenses that scatter the light) and the shadow you cast on the ground goes from being crisp to fuzzy and even appears to vanish.
This demonstrates that cast shadows closer to a light will always be sharper than shadows further away. The wider and closer the light the fuzzier the shadow. That’s why the sun which is more than 25 times larger than the entire earth, casts sharp shadows. It’s very bright but very far away.
That’s all for now!