Light Pictures

We've been experimenting with camera settings and small, different co-loured lights. We found that if we kept the room lights low and took pictures with a long exposure time, we could make our own light pictures.

We even tried writing our names with laser pens - not quite as successfully!

Burning chips!

No - not that sort of chips! Integrated circuit PIC microprocessor chips!

"Look - I've done it! I've burned the Pic programs onto the 12F675!" says Mrs G

Yes, yes Mrs G - well done! But you are just copying our programs onto our hardware systems! We are the ones that did all the clever stuff!  :)

not so bright...

Our red LEDs use about 2V and about 20mA so use about 0.04W (we should measure this really) and for a red led the brightness is about 70 lm/W so a total luminous flux of about 3 lm if it is 100% efficient. A typical efficiency for Red LEDs . It emits most of its light forwards so illuminates, say 1m2 at a distance of 1m (to make the numbers easy!) this corresponds to about 3 cd or 3 lx (they are the same value at a measurement distance of 1m!)

These are very rough numbers, but in a dark room, Luxmeter app on the iphone measured the LED brightness as 1 lx and a candle as 2 lx too so our LED seems to be similar to a candle (although I'm not convinced how dark the room was or the accuracy of Luxmeter at these light levels) 

We would need to measure things more precisely to get absolute light intensity, but we can measure our light source brightness relative to our standard LED for now. 

How can we make it dimmer? One easy way is to switch it off for half the time, and then it will emit only half the light! If we switch it on and off really quickly, quicker than the persistence of vision it will be perceived as just a dimmer source. In practice, this means more than about 60 times per second (60Hz) in bright light or about 10 times per second in dim light (10Hz) due to the difference in integration time between cones cells (10-15ms) and rods (100ms). 

We can do this by programming a microchip PIC 12F675 chip to control the LED. We can use JAL a simple programming language to write a program and then compile it and load it onto the chip, so that when we supply power to the chip it flashes the LED. 

Here is an example program that we can modify to make our LEDs flash!

Actually, the only bits we need to modify are the delay statements (giving delays in microseconds) and the instructions to set the LED pin high (LED on ) or low (LED off) - easy! ...and here's the hardware circuit!

(much easier circuit than our flashy LEDs because we did all the work programming the chip and don't need lots of different components!)

To start with, we will just make them flash slowly to make sure everything works, and to flash our name in morse code (because we can!)

Bright lights!

We know we see very faint lights in black and white rather than colour (stars all look white apart from Betelgeuse and Aldebaran) and that really faint things can be seen better out of the corner of our eye - our theory is that this is because the cones cells in the eye see colour and rod cells in the eye cannot differentiate colour but are more sensitive to low light. 

We can test these ideas in our field of vision test box! We have started to test the angular field of view (results later) but if instead of just using standard LEDs as the light source at different angles, we control the colour and brightness we can investigate how faint a light we can see and when we stop being able to see colours at different angles...

To do this we have two problems:
1 - How bright is a light? how do we measure brightness?
2 - How can we make a faint light and make lights of know brightness (or at known relative brightness)

First (we'll deal with the second issue in another post) what do we mean by brightness? Bright enough to see by? Bright enough to read by? An obvious (to a physicist!) definition would be a measure of how much light energy a light gave out, but that is complicated because different colour photons have different energies (ultra-violet photons have much more energy than red photons for example, but we cannot see in ultra-violet so energy by itself is not very useful for measuring brightness). We could count the photons themselves, which would be better, but would not help us define a bright light because again, we do not see all photons equally - the cone and rod cells have different ranges of sensitivity (and again do not respond to ultra-violet or infra-red photons...). What we want to measure is how many visible photons come from a given source or area and weight this by how sensitive our eye is to them. 

In fact this makes it easier - we just need a standard light and compare all other lights to that!

Scientists wanted to do this a long time ago so they picked the most obvious standard light for the time - the Standard Candle and this remains (more or less) the official unit for luminous intensity (brightness) or amount of light emitted per unit solid angle.

There are some other units that are important when quantifying how bright something is...

The further away the candle is, the fainter it appears because the light is spread out - we are actually more interested in how much light is arriving in our eye than how much is given out by the light. We define the Candela (cd) in terms of a rather precise type of light source now, but a standard candle emits light with an intensity of about 1cd and this is a measure of the energy of visible light per unit solid angle - this is the luminous intensity of the light. 

We measure the total amount of light given out by a light source, the luminous flux (measured in Lumens, lm), by the luminous intensity (measured in Candela, cd) multiplied by the solid angle over which it is emitted. Finally, we measure the amount of light arriving at a surface (measured in Lux, lx) in lumens per unit area (!) All of this has to be defined in terms of a frequency of visible light and related to the sensitivity of the eye! It's complicated!

So if you are comparing light bulbs to buy, you are interested in how many lumens they give out, if you are interested in whether you can see a faint light you are interested in how many lux it gives at your eye.

We tested this initially last week and used a iPhone Luxmeter app to measure the brightness of a candle (standardish!) in Lux and compare to one of our LEDs but I lost the results! Oops - we'll have to repeat it...

UPDATE!
Some of us remembered the LED and the candle both gave readings of about 1 lx at a distance of 1m (reassuringly the candle gives out 1 cd!)

As memory is not very infallible, I repeated it it in the darkest room at home and got 1-2lx for the candle and 1 lx for the LED - the candle gave 4lx ish at 0.5m (it should be 4 times more lux at half the distance) so it looks like our LEDs are similar to standard candles given the uncertainties associated with Luxmeter at low light conditions and the problems of background light and reflections....)

Triangles in the Blind-spot Box

Our new blind-spot measurer allows us to read off how far the eye is from the test card when the spot is in the blind spot, but what is the angle? It would be inconvenient to have to get a protractor out each time and measure the angle directly!

Fortunately, there is a simple relationship between the lengths of the sides of triangles ad the corresponding angles! This has been studied since ancient times (pyramids, timings of sacrifices etc…) and is now known as trigonometry. 

If you look at right-angled triangles with the small angle on the left...

If you put together 4 small triangles you make one big triangle that has all the same angles, and the sides are exactly twice as long. This means that for the big triangle the ratios of the sides are the same as for the little triangles. We can add more little triangles to make bigger and bigger triangles and the ratio of the upright side length to the bottom side length will stay the same. In fact it is the same for any triangle with these angles. So if we know the angles of a right-angled triangle, we know what the ratio of its sides will be (so long as we have measured that ratio on any similar triangle). Likewise, if we know the ratio of the edges, that will determine the angle.

This is just what we want! We know that the separation of the dot and cross is 7.4cm, and we measure the distance from the eye to the cross (it is marked on the box and add 1cm for the distance of the eye behind the window) so we can calculate the ratio. 

All we need is for a list of the ratio of these sides for every possible angle!

Fortunately (again) someone has done this for us! Islamic mathematicians invented the field of trigonometry in about the 9th century, and made tables of the ratios of triangle sides for all possible angles. These are SINE COSINE and TANGENT functions and you learn it in GCSE maths because it comes in useful in lots of areas of science. (Islamic mathematicians invented lots of the basic concepts of mathematics and lots of the words used are derived from Arabic).

Here’s the table we need:

Now all we need is 300 volunteers to measure!

Big Fish - Little Fish -

We've been building the experimental equipment...

Easy test equipment for the rest of the school to measure their blind spot angle - they look through the hole and slide the test card back and forth until they find the point at which their blind spot hides the dot - then we read off the distance and make a look table to find which angle that corresponds to! Quick and easy! We'll bribe them with a thinky so they do the experiment for us!

(Here's thinky the monocular-illusion-dragon! (look at him with one eye closed... ) 

We made lots of him and everyone except those with an over-intelligent visual cortex thought he was very cool/wierd)

Field-of-vision test box - we look through the hole in the middle and we will position LEDs poking into the box that we can light up and see if we see them whilst we look straight ahead. We will be able to measure field of view and sensitivity to how bright they are and sensitivity for different colours :)

Fair enough!

We need to be sure that our random flash LED light really is random or else sneaky brained visual cortexes will keep checking over where the light flashes most often, and we need to know if we are going to use it as an electronic dice for gambling of course! (Not encouraged by the 'Out of Sight' Blog or the Royal Society of course).
Lots of us did a tally count test of lots of blinks...

I've put the results in a spreadsheet to make it easier to see :)

Is that what we expect? It is difficult to say exactly - you could just be very lucky and keep rolling sixes even if it is uniformly random, but there is a test you can do which tells you the probability that you would get results this different from the ideal equal proportions of each number - the Chi-squared test. You can see that there is a bigger than 10% chance that our results would be at least this uneven for most individuals and for the combined total - student 6 looks to be a bit odd though... 99.5% probability that data like theirs isn't generated by a fair circuit! Maybe the capacitor was big so their LEDs flashed slowly, and then if they were testing quickly the LEDs selections would not be independent (or maybe they didn't like 5s and cheated?)