Hi Rifat Haidar ,

The best thing to do is to enter the specifications of the tube, lens, etc. in the Zemax or COMSOL Multiphysics software and design the best conditions.

Best regards.

Hi, i dont have that software. a schematic diagram of optical arrangement would do for now.

What wavelengths are of interest?

* What's the angular acceptance angle of the sensor?

* Can you support lenses freely some distance from the tube?

* This is all happening in air or a vacuum, right?

* Is there a reason to not have the sensor viewing the heated vessel, rather than relying on an IR-transparent fibre?

I've used ZnSe lenses to focus four micron IR quite nicely in the past, using pyroelectric and thermopile devices.

the wavelength of interest is 650-800 nm.

Rifat,

Okay - so the sensor is an infra-red photodiode or similar?

Are there reasons that you have to use an IR fibre to conduct the light?

Could you insulate the heck out of the sensor, or put it on a cooled copper finger, and have it directly view the aperture?

Hi, I sent you personal message. No I cannot insulate the sensor.

Compatibly with the minimum working distance allowed between the tube and the sensor, you could place a lens between the hole and the sensor, as close as possible to the hole and having a focal length such that the active area of the sensor will coincide with the real image of the hole. For example, if assuming hole and sensor having the same size, you could put a lens of focal length F at a distance 2F from the hole, and the sensor at a distance 2F from the lens to obtain the image 1:1 of the hole on the sensor. If instead you were forced to use an optical fiber, and if you cannot put the fiber very close to the hole, you should do the same, but considering the input aperture of the fiber instead of the active area of the sensor. Anyway, if using the fiber, you will place the sensor very close to the fiber exit aperture, or should use a second lens to image the fiber exit aperture on the active area of the sensor.