aantonio

Maybe I am wrong here, but I think in a fisheye lens only the given f/D (by manufacturer) matters, not the actual diameter of the glass we can measure. The particular shape of such lenses is needed for 150-180 degrees pictures, anyway not all sun light incident on the "apparent glass diameter" is actually sent to focus. (otherwise I fail to understand the meaning of data like f/D and f, stated by lens manufacturer) The light concentrated on the chip should then follow my original estimate if I choose to believe to the f/D given by the manufacturer: intensity at focus = 0.15 W/cm2 / (0.5 deg / 57 deg/rad)^2 / (f/D)^2. Even assuming f/D = 1 (!) this means 2 kW/cm2, not 25 kW/cm2 which looks too close to expected damage threshold... Few kW/cm2 seems to me more consistent with empirical evidence of many allsky cams happily exposed to sunlight everywhere, what do you think? Antonio PS: 1 cm2 lens area and 20 um sun "approximate square" size would mean f/D = 0.2 (I think there is a problem with these numbers)

There is another important point I quickly mentioned in my original post and in my reply to Pete, related to heat diffusion (conduction). Damage threshold corresponds to a critical temperature increase. Light intensity (heat source) is limited to about 1 kW/cm2 for all lenses, however the temperature increase is related to “f” (rather than “D”), being proportional to heat source and thermal diffusion time constant. The latter depends on the square of the minimum size of the irradiated semiconductor volume. Increasing sun disk image with longer focals will increase the size of the irradiated zone, therefore the thermal resistance for the heat path, resulting in higher central temperature and easier damage. Thus sensors of consumer digital cameras equipped with focal lengths longer than 10s mm might suffer from direct sun exposition, certainly more than small webcams or smartphone cameras! This same property of heat diffusion makes burning paper with a short-focal lens even more difficult! I hope this will make things clearer! Antonio

Exactly, Pete: thank you. This is the article I wanted to link! As for the intensity at lens focus, my result is correct. What part of my calculation you don’t understand? Maximum possible value is around 1 kW/cm2, which is much less than 50 kW/cm2... and yes, a 2-mm focal lens would produce a sun image of just 20 um! Antonio

No, Pete. Numbers are right. Here is the paper mentioned: shorturl.at/djLRZ I know, it's hard to believe even 1 kW/cm2, but that is true. In calculating temperature, you don't simply put all that power into a closed volume. Remember Fourier diffusion equation: this helps spreading heat, the smaller the spot, the faster! At the lens focus there is really that power density, no cheating. The article says they have used exposures as long as 10 sec, while direct exposure to sunlight for a single pixel may last as long as 1 hour. This does not change the picture, since temperature has reached steady state through diffusion. Time constant is shorter than 10 sec. Antonio

Hello, this is my first public post in this forum. I am one of the “robotic” guests at Olly Penrice fantastic place, since 2016. I found valuable information and creative ideas here on SGL when setting up my all-sky webcam based on a ZWO ASI120 mono. In return I’d like to post something that might reassure those who are afraid of leaving their webcams permanently exposed outside, pointing skyward and potentially imaging the “super-bright” sun disk on their poor sensor! Although initially I was also fearing such situation, a deep internet search showed that apparently there is no risk, confirmed by a trusted fellow expert astro-imager at Olly’s. But I needed to understand why… Let’s calculate the intensity (W/cm2) at the sun image on the focal plane, as this is responsible of damage if it exceeds some critical value. Power collected by the lens with diameter D is = sun intensity on Earth surface (about 1.5 kW/m2) x aperture area (pi x D^2 / 4). Focal spot diameter = sun angle (about 10 mrad or 0.5 deg) x focal length f. Result: focussed sun intensity on sensor = 0.15 W/cm2 x D^2 / (0.01 x f)^2 = 1.5 kW/cm2 / (f/D)^2. Therefore intensity depends only on (f/D): in worst case, say f/D=1, such intensity is about 1.5 kW/cm2. This may sound a lot, but… I was puzzled and wondered why only reflex cameras etc. (with larger lenses) are routinely reported to suffer damage by sunlight (not unexpectedly, mostly during eclipses!)… Also, why is it harder in practice to burn paper with a 10-mm diameter lens compared to a 50-mm lens? (assume similar f/D) Apparently the diameter does seem to play a role eventually, but… it’s not so! The dilemma was solved when I started to look at the right problem: “intensity damage threshold of ccd / cmos”. (nothing specifically related to sun damage, but rather to laser damage occurring in imaging equipment in photonics industry) I stumbled across a nice scientific paper reporting detailed measurement of laser damage threshold for CCD and CMOS. Guess what? They measured a damage intensity of about 50 kW/cm2 under continuous laser irradiation at 532 nm. The fact that sun light is not monochromatic is not important: in this case, light is only used to produce heat. Conclusion: all-sky sensor are perfectly safe, as I learned earlier from empirical evidence! Why are more sophisticated digital cameras prone to damage then? If you look at damage pictures, these pretty much consist in failure of something else than the sensor: filters, iris blades, plastic parts or anything that can heat and burn in sunlight. These can release debris that stuck on and can permanently damage the sensor itself. An all-sky cam captures most of the “upward” sky, where the sun is brighter. When it is dimmer, closer to the horizon, even if it focuses outside the sensor it does not harm the camera. Finally, since the sun disk image on the focal plane is given by the angle (10 mrad) x focal length, a 10-mm lens will produce a sun spot of 0.1 mm, to be compared with a 0.5-mm spot for a 50-mm diameter lens. Try to keep the paper in focus, and the focus steady with 0.1-mm precision for 1-2 sec, with your hands! I hope you might have found this as funny as it was for me to figure it out! Cheers, Antonio