Ambient lighting and visual height perception

[Genghis the Engineer]

I had an interesting experience last week. I flew an air test in a light open cockpit aeroplane (type irrelevant) on a particularly brilliant and sunny day. Whilst I flew it by the altimeter, my perception was that I was much lower than the altimeter said - roughly speaking I was at 800ft on the altimeter, but visually I judged myself at around 500ft.

A had to fly a second sortie a few hours later in the same aircraft, when the circuit was rather busier. With 800ft on the altimeter, I was level with several other aircraft in the circuit who should have been at the same pressure altitude (or height anyway, flying on QFE), so I stopped worrying about it.


A couple of days later I had occasion to fly my own aircraft a couple of times on the same day. During very bright sunlight (middle of the day) I got the same perception of being rather lower than indicated by the ALT. Towards dusk, the perception seemed about right.

I suppose it's possible that some atmospheric condition was causing a massively nonstandard pressure degredation rate, but it seems unlikely. So, I'm left thinking that the bright overhead sunlight modified my depth perception when viewing the ground below me.

Is this a known phenomenon anybody?

G

[gingernut]

Not sure about bright light, but several studies have shown that perception of lit objects is reduced by about 60%, (things appear further away), when viewd through a fog chamber.

[Flyin'Dutch']

It is a well known experience that on very clear days distances seem a lot shorter and the same thing happens at night when you are flying on a clear night. Towns lit up seem very nearby.

[Loose rivets]

Hi Genghis

I would not have thought that with your experience you would not have been easily fooled by natural phenomena, even though some sun and cloud combinations can be a little surrealistic sometimes.

Just a thought. Have your vision checked and compared with a say, one year old prescription. Or, as a technically minded person you could devise a test with a pair of ready-readers, to see if there is any change in the relative focal length between the two eyes. Of course, this will assume that you are familiar with your needs over the last year or so.

However, if you suddenly find a pair of say, +2s that were fine a while ago, and now you need a +2.5 in ONE eye, it might fool you a bit. I understand that it can be very disorientating to a small minority of people.

Should you find that there is a change in one eye, let us know.

[Genghis the Engineer]

I'm unconvinced it's my eyes - other than as a generic property of the human eye (not I'll confess a subject I'm an expert on, I studied optics in my physics A-level a couple of decades ago, but not since and don't really have much feel for the subject).

I've never needed any form of vision correction, have always been able to read the bottom line of the chart at my annual medical. In a brief experiment this morning each eye can focus similarly from about 8" to infinity, and my resolution is good enough to read 1.7mm high italic text (the smallest that happened to be on my desk) at arms length without straining (again, with either eye).

So, I still think personally it's to do with the lighting conditions.

G

[Loose rivets]

Well, sounds like you have fabulous eyesight, but did you spot the double negative in the first line of my reply?

The trouble with eyes, and indeed every thing else to do with the human body, is that aging comes in cruel lumps. However, as long as you are satisfied that the focus is still balanced, you can forget that I guess. Furthermore, the difference has to be quite marked to start getting perspective errors.

I could always read the bottom line on the 6 monthlies, but remembering "custom, on the first day of every" helped a lot with the near focus.

[Genghis the Engineer]

"Aging comes in cruel lumps", a very true statement.

I did say that I don't understand optics much, but yes I am pretty convinced that there's not a great deal different between my eyes.

Plus, wouldn't it have been the case that if there was a difference, modifying my depth perception, that the problem would have been similar regardless of lighting conditions?

G