Hi all!

Long time no see or hear. Hope this finds all the usual suspects alive and well.

Been on extended "leave" from the old Serif community - no not jail time! Other reasons have directed efforts elsewhere. Alfred kindly joined me up. Thanks.

Anyway a nice puzzle for you all right up Jack's street I guess.

I have a IPEVO document camera which connects to Macbook via USB and runs Visualiser software which can show the continuous camera output and enable you to do stuff in real time with the image. Also can connect from an Ipad to a mobile phone in a similar fashion.

Problem: Using the setup last night the laptop camera picture was virtually unusable because of extreme flickering - rolling horizontal thick lines. Would seriously be detrimental to some users. The same also occurred using iPad connected via wireless to iPhone. Image on iPad also suffered from flicker.

I ran same setup on iMac in a small bedroom with no issues whatsoever.

Tried again today this morning and no problem with flicker. Seemed to have nailed the reason down to presence lighting! In the problem environment I have 2 LED ceiling light clusters with bulbs pointing in the direction of the laptop/camera setup and also at the iPad. There is no direct light on the setup when in the bedroom as the iMac is in a enclosed area.

I have tested this afternoon in the downstairs room with curtains drawn and lights on and flicker occurs. Lights off no flicker. So I am hoping that the issue has been identified. Einstein's famous quote "Repeating the same experiment expecting eventually to get a differenet result is a sure sign of madness" suggests I should stop switching lights on and off!

Question: Can anyone explain (Jack?) in simple terms to a pure mathematician with a basic UK A-level Physics knowledge in the very dim distant past whether I have identified the issue correctly and why it occurs. I am very aware that PAL and refresh rates are possibly involved as I have seen references to that and I have also seen multiple times the rolling flicker effect when viewing a TV working through another TV and often wondered why.

Over to you folks.

Good to be back!

John B