Imaging UV light with CCD Cameras

Author: 
Chuck Reagan, Southeast Sales Manager, Ophir-Spiricon, LLC

 

Is it possible to image a UV laser with a Silicon Sensor CCD camera offered by Ophir-Spiricon? The answer is yes, but the direct UV light ablates Silicon CCD chips over time. The ablation is cumulative and depends on the intensity, the wavelength, and the duration of the light on the sensor. The best choice for imaging UV light without damage is to avoid directly imaging the beam on the CCD sensor by using an UV image converter.

Not all customers want to use an image converter or attenuation as it is possible to introduce aberrations or defects in the beam. Those defects may slightly change the beam measurements and displayed profile, which could introduce some uncertainty in the displayed results.

Ophir-Spiricon offers a number of tools and we work very hard to try to avoid introducing error into the measurements. We choose very good optical materials and we reject materials that do not pass our inspections. Several of those tools are shown below, but the first let’s discuss direct UV beam imaging, including attenuation.

Direct Imaging
When imaging direct UV light, the first priority is to protect the sensor from powerful, short-term UV light, which can immediately damage the sensor (just as Visible and IR lasers do). If users are unable to reduce the energy or power of the UV laser with the operator power controls, they may still have to use UV grade, Fused Silica beam splitters and UV grade attenuators, or other acceptable attenuation, which will reduce the energy or power of damaging beams.

After attenuation, the operator should ensure that enough usable UV signal is available to the camera for profiling needs. Many UV attenuators are reflective so users must plan to avoid or contain those reflective beams. Avoid using any beam near the saturation level of the camera. Ophir-Spiricon recommends using image converters for wavelengths below 350nm. Most camera manufacturers place a cover glass over the sensor that blocks UV light below 400nm, which helps protect cameras.

Ophir-Spiricon removes that glass from most of their cameras to allow customers the option to use that area of the response curve if they need it. Having no cover glass on the camera CCD sensor opens the possibility for mechanical damage to the sensor. The normal sources of mechanical damage include a blast of shop or canned air, fingers, or a cotton swab, which will guarantee damage to the sensor.

Below is a 248nm laser directly imaged onto a CCD.
 
248nm laser directly imaged onto a CCD

Damage Limits
Many people ask, “How long will it take and what power/energy settings will damage my sensor by slow ablation if I use it daily or weekly with direct, even attenuated, UV light?” Due to the lack of published directives or limits, the answer to that question is unknown. Manufacturers do not provide CCD ablation charts, maximum allowed energy/power charts, duration-of-exposure-to-damage-charts, or any other named limits charts or tables. Even if using maximum allowable attenuation, the UV light will still eventually ablate the sensor if used for long periods of time.

Indirect Imaging
As stated above, Ophir-Spiricon recommends using image converters for wavelengths below 350nm.

 

Those who require that approach should discuss their options with an Ophir Sales Engineer to review the tools available and the damage threshold of those devices. Below are some examples of good tools made by Ophir-Spiricon, which can make the job easier. Indirect Imaging

Image Conversion Plates and Lenses
Ophir-Spiricon offers 50 X 50mm square (and 100 X 100mm) UV image conversion plates for use with a focusing lens. The lens is mounted to the camera and the plate is placed at a convenient location in the beam path for imaging the beam. The plates are not for focused beams and the beam intensities must be from 1mJ/cm2 to 20mJ/cm2. These UV plates are not suitable for 193nm. The 50mm lens is part of a kit which includes C-Mount spacer rings.

Integral Reimaging UV Image Converters
Other tools that can be very useful are the Integral Reimaging UV Image Converters; several examples are shown below. These beam expanders and reducers are equipped with UV converters; extra beam splitters are available. These converters operate from 193nm up to 360nm.

Integral Reimaging UV Image Converters

Integral Reimaging UV Image Converters

Attenuation Tools
Below is an example of Ophir-Spiricon C-Mount UV filter. The Attenuator assembly is mounted to a BeamGage CCD camera.

Attenuation Tools

Attenuation Tools

These UV Filters have a damage threshold of 100W/cm2 CW, and 20mJ/cm2 for nanosecond pulse width lasers, allowing no distortion for the UV filters. If you need more attenuation, we offer several uncoated UV-grade Beam Splitters that can lower the power/energy of your UV lasers.

Below is a version of the LBS-400 mounted to a Pyrocam IV, UV, and IR camera. Ophir-Spiricon offers a UV version of this dual-beamsplitter and UV filter assembly with a 1.25 inch clear aperture.

Attenuation Tools

Ophir-Spiricon also offers a 355nm, C-Mount screw-on filter if you wish to image the 3rd harmonic of YAG. This filter transmits 355nm but blocks 532nm and 1064nm

Attenuation Tools

Attenuation Tools

Below is a UV grade, C-mount dual beam splitter useful from 200nm up to 2.5um.

Non-Camera Based UV Profilers: NanoScan2
In addition to cameras, Ophir-Spiricon offers the NanoScan2, scanning slit profilers, which are useful for analyzing many types of UV beams (below).

 

Ophir-Spiricon offers many other useful tools for imaging UV laser light that are not listed here. Contact your sales engineer for application details. Non-Camera Based UV Profilers: NanoScan2

 

Non-Camera Based UV Profilers: NanoScan2

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