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Most of the reflective sensors in the market use either red or near IR wavelength and have a very short range (in the range of mm). Given the reflectance of the Si at different wavelengths and reflective material used for reflection, i need to select wavelength of the light for sensing long distance in dark environment(in the range of 10 - 20 cms). How does the distance between the sensor and the sensing object be taken as a factor in selection of wavelength?

As per my research, energy of light is based on intensity and wavelength. Intensity of light is inversely proportional to distance and the energy is inversely proportional to the wavelength.

KPK
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    Your premise appears to be faulty - visible light sensors using reflectors work pretty well to several metres. – Andy aka Apr 13 '20 at 12:06
  • "*Intensity of light is inversely proportional to distance ...*" No, for an unfocused beam the intensity of light is inversely proportional to the **square** of the distance. This means that as the distance from a light source increases, the intensity of light is equal to a value multiplied by **1/d²**. Industrial sensors detect over many meters by using focusing lenses to prevent the beam diverging. – Transistor Apr 13 '20 at 12:18
  • does your TV remote control only work in a dark room and only at a distance of 10 to 20 cm? – jsotola Apr 13 '20 at 16:00
  • What exactly do you want to sense? Presence? Distance? Location? – Elliot Alderson Apr 13 '20 at 17:59
  • I want to detect presence of metallic object (Eg. Aluminium) over a distance of 10cm using reflective sensing. – KPK Apr 14 '20 at 03:59

2 Answers2

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You can look at the output (when converted into some common units) for various LEDs or lasers and compare the sensitivity of various sensors.

Optimum is probably IR as used for TV, A/C etc. remote controls and optoisolators but I don't think there is a huge difference (especially with highly optimized modern visible light LEDs) unless you get to very short or long wavelengths such as blue or UV and far-IR.

The reflectivity of the target probably makes a much bigger difference. Retro-reflective targets such as the material used in highway signs or corner reflectors have been shown to work well in practice with distances up to about \$3.8\times10^8\$ meters.

Spehro Pefhany
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    One problem with using visible light: If the environment is dark for a reason other than making it easier to use optical sensing, and you use visible light sensing, then it won't be dark any more. (Depending on the reason for keeping it dark, the same might apply to IR sensing) – The Photon Apr 13 '20 at 15:57
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Most of the reflective sensors in the market use either red or near IR wavelength and have a very short range (in the range of mm).

IR (really NIR) is commonly used because it is cheap and people will not notice the light, which could be annoying if it were visible. Range is not limited to millimeter however. I have NIR room sensors that are quite effective at 10m range, and people have measured the distance to the moon (385,000 km) using NIR lasers. Optics, not wavelength, determine the effective range.

How does the distance between the sensor and the sensing object be taken as a factor in selection of wavelength?

More power will give you longer range, but optical power is limited by safety. Using NIR (or better, IR) wavelengths will enable you to use more power while remaining eye-safe.

Edit

Using IR wavelengths absolutely does allow you to use more power while remaining eye-safe. There are two reasons for this. First, photochemical effects only happen at visible wavelengths and below (and so an entire damage mechanism does not occur in the IR). Second, the absorption of light by the retina and the transmission of light by the vitreous humor both decrease as you move from the visible to IR spectrum. Hence more IR power must be applied to have the same dosage absorbed by the retina. I'll go through and address the replies individually.

-1 for claiming that using an a non-visible wavelength at higher power is just as safe as a visible wavelength at lower or equal power.

The ANSI Z136.1 specification defines maximum permissible exposure powers as a function of wavelength and exposure time:

MPE-Z136.1-2007

Moving from the visible to 1050nm for example, the MPE increases by a factor of 5 for short (thermally-limited) exposure, to up to a factor of 50 for long (photo-chemical limited) exposure. 50 times more power is a huge difference in eye safety. There are no IR wavelengths where the exposure is lower than visible, so IR will always allow more power while remaining eye safe.

Visible light actually can have higher limits for eye safety, because the aversion response (you won't keep staring at a visible source like you might with an IR source) is taken into account.

According to Z136.1, eye safe refers to class 1 devices (see Appendix H), so the aversion response is not considered when determining if a device is eye safe. Instead, you use the MPE directly, and you get that the exposure limit is lower in the visible.

This makes sense. The purpose of the exposure limit is to figure out if a device can cause injury. The aversion response can mitigate injury if a non-eye safe source is briefly viewed, but you first need to figure out if the device is capable of causing injury before that matters.

user1850479
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  • -1 for claiming that using an a non-visible wavelength at higher power is just as safe as a visible wavelength at lower or equal power. – DKNguyen Apr 13 '20 at 15:55
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    Visible light actually can have higher limits for eye safety, because the aversion response (you won't keep staring at a visible source like you might with an IR source) is taken into account. – The Photon Apr 13 '20 at 15:59
  • @DKNguyen The MPE limits in the visible are lower than in the IR, so that is a factually accurate statement. See: https://en.wikipedia.org/wiki/Laser_safety#/media/File:IEC60825_MPE_J_s.png – user1850479 Apr 13 '20 at 17:11
  • @The Photon If you are designing a sensor to be eye safe, you typically design it to be safe for long term exposure. 8 hour exposure is a common value. For exposures measured in hours, the aversion response is irrelevant. If a device depends on the aversion response to avoid retinal injury, it is absolutely not eye safe. – user1850479 Apr 13 '20 at 17:16
  • @user1850479, the exposure time isn't chosen arbitrarily, it's defined by IEC, ANSI, and FDA standards. A visible laser (with various other considerations) can qualify for class 2 statusat up to 1 mW or class 3R status at up to 5 mW due to aversion response, where an IR laser can not. – The Photon Apr 13 '20 at 17:33