A headlamp aligner is a box with a lens and a screen that you roll in front of the lamp, and the beam draws its own cut off line on the screen so you can measure where it points. Roll it in front of the nearside lamp of one car with the bulb in my hand, and the screen shows a beam, cut off and all, at the right height.
I am an MOT tester and the headlamp aligner is the last check on the lane.
How it works.
The aligner is a box on rails with a big lens on the front and a screen inside. You line it up with the lamp, the lamp shines into the lens, and the beam's pattern appears on the screen, smaller, with its cut off line, which is the sharp edge between light and dark that a dipped beam has. The screen is marked in percent of dip, and the cut off has to fall in the band.
There is no electronics in the old type. It is a lens and a screen. If there is no light going in there is nothing on the screen.
One car, a hatchback that is tested here every year, shows a beam on the screen from its nearside lamp with the bulb removed.
I found it because the bulb had gone. The lamp was dark, plainly, to the eye, and I rolled the aligner in front of it out of habit before changing the bulb, and there was a beam on the screen. Cut off, kick up, the right shape, sitting at minus one point two percent, which is a pass.
The bulb was in my hand. The lamp housing was dark. I could see the reflector through the lens, unlit.
The screen showed a beam.
What has been checked.
The aligner. It is a box with a lens and a screen, and I have had the back off it. There is no way for it to show a beam that is not going into the lens.
Another aligner. The garage down the road lent me theirs, an electronic one with a camera. It showed the beam, and its software measured it and printed a pass.
The lamp. Bulb out, connector off, the wiring to that lamp disconnected at the fuse. The screen shows the beam.
A different lamp. I have swapped the whole headlamp unit for a new one from the dealer. The new unit, with no bulb in it, shows the beam on the aligner's screen at minus one point two.
So it is not the lamp. It is the place where the lamp is on that car.
What the beam looks like on the screen.
The right shape. A dipped beam pattern is distinctive: flat on the offside, kicked up on the nearside so it lights the kerb, and the cut off is a hard line. This is that.
It is not the colour of the car's bulbs, which are halogen and yellowish. It is whiter and it has a faint flicker in it, the flicker of a lamp on a poor earth, or of something older than that.
I have put my hand between the lamp and the lens. The beam on the screen has my hand's shadow in it.
What is done about it.
The car passes its headlamp check with the bulbs in, because with the bulbs in the lamps shine and the aligner reads them and they are aimed correctly.
I have never written down what happens with the bulbs out because there is no box for it and because the certificate is for the car as tested, and as tested it is fine.
What I do.
I do the lamps last, as always.
The one thing I have changed is that I do not test that car's lamps in the dark.
The lane has a roller door and in winter the last tests are done with it shut and the lights down, because that is how you see a beam.
With the door shut, in the dark, with the bulbs out and the wiring off, the beam is on the screen of the aligner, and it is also on the roller door. Not a reflection: a dipped beam pattern, kicked up on the nearside, thrown onto the door from a headlamp with no bulb in it, bright enough to read the sign on the door by, and I walked into it and it was warm on my face.