Near IR webcam

Public summary: 

There's a whole spectra of light beyond the visible, why not see one side of it through a modified webcam?

Use a modified webcam to see into the near infrared.
Useful information
Kit List: 

Small grey box containing:
Webcam (modified)
Lights (Incandescent and compact fluorescent)
Shaver (blades removed)
Paper with CHaOS logo written with different pens
small white remote control

Packing Away: 

Store the laptop (and its power supply/lock) in the appropriate laptop case. All other equipment, including the webcam itself, should be carefully packed into the experiment box.

Frequency of use: 

What it it?

Take a normal webcam, remove the IR filter and replace it with crossed polars or a piece of exposed colour film and hey presto you have a near IR camera. Practical point:- the webcam has a focusing ring in front of the lens, which may be turned during an event, causing the image to become blurred. If this happens, ask a committee member to show you how to focus it (there is a knack, and if you it wrong you can jam the focus).

What is light/ IR light?

The light most of us are really familiar with is white light. But that's actually a mixture of different colours. Most of the time we can't tell that, but sometimes we can see these colours split up, like in a rainbow.

Do you know the colours of the rainbow? Ask them to tell you!
[Red -> Orange -> Yellow -> Green -> Blue -> Indigo -> Violet (basically purple!)]

It turns out that there are more kinds of light than our eyes can see. Some kinds of light are "redder" than red is - we call that kind of light "infra-red" . You might have seen images from cameras that can detect this kind of light, perhaps watching animals at night on TV, or police chase criminals with helicopters. At the other end of the rainbow there are other kinds of light that we can't see. These are more blue/ more purple than violet, and we call this kind of light "ultra violet".

It's weird to think that there's kinds of light we can't see, but not all kinds of eyes detect the same light. For example, some people can't tell red and green apart, which is called "colour blindness". Another example: some insects, such as bees, can see UV light. Some types of flowers have extra patterns in UV, so this helps them find the nectar in the middle of a flower! (How cool is that?!)

But what about wavelength/ spectra?

Add this extra level of detail with caution: it can be too much detail to take in if they've never thought about IR/UV before, and you can overwhelm them. You can come back to this later on once you'rve showed them some of the cool things you can see with the IR camera!

EM Spectrum: we call all the kinds of light (including UV, IR, visible) "electromagnetic radiation". One way of understanding this is to say that all these kinds of light have different sizes of wavelength. There's some charts in the box that you can point at when you explain this. Start with the rainbow: Red light waves are wider/longer than blue light waves. Following on from that, infra-red has longer waves than red light; UV has a shorter waves than blue. If you go further outside that you can see microwaves (that you can cook with) and radio waves (which can hold information. like music). These have a longer wavelength, much longer than infra-red. If you go the other way you get to X-rays- these have smaller waves than UV!

What do I look at with the camera?

Firstly, plug the new camera into a laptop and use VLC player (open up the video devices settings and select USB input owtte).
[Or maybe do ->Open Capture Device (Ctrl+c)-> PC Camera this sometimes also works]
[The camera often lags or gets stuck, the best solution is closing and reopening VLC]

Things to look at:

Coke is transparent, as are some plastics (shaver casing, sample of smoked glass).

Bank notes have lines. Try looking at the Queen's head on a £5 (old ones or new polymer ones both have the same effect!) or £20 note.

CD can be used as a diffraction grating to produce a 'rainbow' - shifted position.

Incandescent vs compact fluorescent - about the same luminosity in visible, former is much brighter under IR (Note: This looks more convincing if you give the fluorescent bulb 30 seconds or so to reach full brightness).

Different materials - some black clothing appears white under IR, often patterns on clothing disappear.

On the paper with the CHaOS logos, one shows up in IR and the other doesn't (written in different pens) - there is also the Normal/IR vision one with the same effect.

The laptop screen appears blank.

Remote control- uses IR, point at the camera while pressing buttons. Many camera phones lack IR filters so can test this on parents phone use it to see light from the end of the remote control.

Crookes' Radiometer

A Crookes' radiometer has four vanes suspended inside a glass bulb. Inside the bulb, there is a good vacuum. When you shine a light on the vanes in the radiometer, they spin -- in bright sunlight, they can spin at several thousand rotations per minute!

The vacuum is important to the radiometer's success. If there is no vacuum (that is, if the bulb is full of air), the vanes do not spin because there is too much drag. If there is a near-perfect vacuum, the vanes do not spin unless they are held in a frictionless way. If the vanes have a frictionless support and the vacuum is complete, then photons bouncing off the silver side of the vanes push the vanes, causing them to rotate. However, this force is exceedingly small.

Over the years, there have been many attempts to explain how a Crookes radiometer works:
Crookes incorrectly suggested that the force was due to the pressure of light. This theory was originally supported by James Clerk Maxwell, who had predicted this force. This explanation is still often seen in leaflets packaged with the device. The first experiment to test this theory was done by Arthur Schuster in 1876, who observed that there was a force on the glass bulb of the Crookes radiometer that was in the opposite direction to the rotation of the vanes. This showed that the force turning the vanes was generated inside the radiometer. If light pressure were the cause of the rotation, then the better the vacuum in the bulb, the less air resistance to movement, and the faster the vanes should spin. In 1901, with a better vacuum pump, Pyotr Lebedev showed that in fact, the radiometer only works when there is low-pressure gas in the bulb, and the vanes stay motionless in a hard vacuum. Finally, if light pressure were the motive force, the radiometer would spin in the opposite direction, as the photons on the shiny side being reflected would deposit more momentum than on the black side where the photons are absorbed. This results from conservation of momentum - the momentum of the reflected photon exiting on the light side must be matched by a reaction on the vane that reflected it. The actual pressure exerted by light is far too small to move these vanes but can be measured with devices such as the Nichols radiometer.
Another incorrect theory was that the heat on the dark side was causing the material to outgas, which pushed the radiometer around. This was effectively disproved by both Schuster's and Lebedev's experiments.

A partial explanation is that gas molecules hitting the warmer side of the vane will pick up some of the heat, bouncing off the vane with increased speed. Giving the molecule this extra boost effectively means that a minute pressure is exerted on the vane. The imbalance of this effect between the warmer black side and the cooler silver side means the net pressure on the vane is equivalent to a push on the black side and as a result the vanes spin round with the black side trailing. The problem with this idea is that while the faster moving molecules produce more force, they also do a better job of stopping other molecules from reaching the vane, so the net force on the vane should be the same.

The greater temperature causes a decrease in local density which results in the same force on both sides. Years after this explanation was dismissed, Albert Einstein showed that the two pressures do not cancel out exactly at the edges of the vanes because of the temperature difference there. The force predicted by Einstein would be enough to move the vanes, but not fast enough.

The final piece of the puzzle, thermal transpiration, was theorized by Osborne Reynolds in an unpublished paper that was refereed by Maxwell, who then published his paper which contained a critique of the mathematics in Reynolds's unpublished paper. Maxwell died that year and the Royal Society refused to publish Reynolds's critique of Maxwell's rebuttal to Reynolds's unpublished paper, as it was felt that this would be an inappropriate argument when one of the people involved had already died. Reynolds found that if a porous plate is kept hotter on one side than the other, the interactions between gas molecules and the plates are such that gas will flow through from the cooler to the hotter side. The vanes of a typical Crookes radiometer are not porous, but the space past their edges behaves like the pores in Reynolds's plate. On average, the gas molecules move from the cold side toward the hot side whenever the pressure ratio is less than the square root of the (absolute) temperature ratio. The pressure difference causes the vane to move, cold (white) side forward due to the tangential force of the movement of the rarefied gas moving from the colder edge to the hotter edge.

There are lots of cool things to look at according to this:

Link to UV

This experiment often links well if placed near UV or demonstrated as a pair. You'll find the lights in IR can be too bright to see UV florescence. If separate demonstrations try and place slightly further apart or use the boxes as a screen if paired then switch off when moving across.

Risk Assessment
Date risk assesment last checked: 
Tue, 01/01/2019
Risk assesment checked by: 
Grace Exley
Date risk assesment double checked: 
Tue, 01/01/2019
Risk assesment double-checked by: 
Risk Assessment: 

Using a webcam sensitive to near infrared light to look at various objects.

Hazard Risk Likelihood Severity Overall Mitigation Likelihood Severity Overall
Lightbulbs (glass) If the lamps are knocked over, the bulb may shatter and cause cuts. 3 3 9 Take reasonable level of care with lamps. Do not place near desk edge. Prevent children playing with lamps.
Call first aider in case of injury
2 3 6
Incandescent light Incandescent light gets hot if left on, causing burns or possibly fire. 4 3 12 Do not allow children to touch lamps, do not place too near the darkroom wall, or any flammable object. Turn off between demonstrations.
In the event of broken glass, move public away and clear up mess carefully as soon as possible. In the event of injury, call first aider.
3 3 9
Fluorescent bulb Compact fluorescent bulb contains (very small) quantity of mercury. 4 4 16 Ensure lamp is stable and not easy to knock over.
If lamp becomes broken, keep the public well away from the area, and ventilate area where breakage occurred. Take usual precautions for collection of broken glass. Do not use a standard vacuum cleaner for cleaning up dust; instead, pick up pieces/dust with a damp cloth or damp paper towels. Place materials, including the cloth/towels, in a sturdy closed container to avoid generating dust. After you have picked up all that you can, then vacuum the area. Finally, ventilate the room where the breakage occurred. Call first aider in case of injury
3 3 9
Lightbulbs turned on in a darkroom Lightbulbs can appear very bright when just switched on in dark room. Eyes are not used to that brightness, so children may be dazzled. 3 2 6 Warn children/visitors not to look directly into the lamps when you switch them on.
Call first aider in case of injury
2 2 4
Electrical cables Trip hazard. 4 3 12 Try to keep cables out of thoroughfare. If cables must be placed somewhere people are likely to be walking, tape them down.
Call first aider in case of injury
2 3 6
Out-of-date Coke Children may drink the Coke – possible stomach upset. 3 3 9 Do not let children drink the Coke. In the event of drinking, call first aider. Coke bottle is sealed with tape. 2 3 6
Crooke Radiometer May fall and smash - possibility of cuts 2 2 4 Keep excitable children away from the radiometer. Don't let them touch the radiometer - there's no need to. Keep it in a safe place, either in sight, or away. If the radiometer does smash, clear up immediately and clear the area until safe. 1 2 2
This experiment contains mains electrical parts, see separate risk assessment.
This experiment is sometimes run in a darkroom, see separate risk assessment.
Publicity photo: 
Experiment photos: