Laser protective plate OD value test

Laser protective plate OD value test

1. Basic concept of OD value

OD (Optical Density) is an indicator that measures the attenuation capability of laser protective materials towards laser light of specific wavelengths. The 

higher the OD value, the stronger the protective plate's ability to block laser light. The calculation formula is: OD = -log₁₀(T), where T represents the transmittance.

II. Selection of Test Wavelength

The testing needs to cover all laser wavelengths that the protective plate is nominally designed to protect against. Common wavelengths include: 1064nm 

(Nd:YAG), 532nm (frequency-doubled green light), 355nm (ultraviolet), 10.6μm (CO₂ laser), etc. Each wavelength needs to be tested separately and the 

corresponding OD value should be noted.

III. Requirements for testing equipment

Laser light source: wavelength accuracy ±1%, power stability better than ±2%

Power meter/energy meter: Its measurement range covers the OD value range to be measured, with an accuracy of not lower than ±3%

Integrating sphere: used to collect transmitted light and avoid loss due to scattered light

Aperture: Controls the diameter of the light beam, typically set to the nominal minimum applicable spot size of the protective plate

IV. Test environmental conditions

The ambient temperature is 23℃±5℃, and the relative humidity is ≤75%. The sample needs to be placed in the environment for at least 4 hours before testing. 

Avoid strong light interference, and the recommended background light intensity in the laboratory is below 100 lux.

V. Test steps

Sample preparation: Clean the surface of the protective plate and inspect for any scratches, bubbles, or impurities. Record the sample number, batch, and 

thickness.

Baseline measurement: Without placing a sample, measure the incident laser power P₀, and take the average of 5 consecutive measurements.

Transmission measurement: Place the sample vertically in the light path and measure the transmitted power P₁. Similarly, measure five times in succession 

and take the average.

OD calculation: Calculate the result according to the formula OD = log₁₀(P₀/P₁). If P₁ is lower than the minimum measurable value of the power meter, it is 

necessary to use a probe with higher sensitivity or reduce the incident power for retesting.

VI. Key points of multi-wavelength testing

For broad-spectrum protective plates, at least three characteristic wavelengths should be selected within the nominal protective wavelength band for testing.

 If the protective plate is labeled as providing continuous wavelength band protection, scanning tests should be conducted at intervals of 10nm or as specified

 by the standard.

VII. Verification of damage threshold

After the OD value test, a higher power laser is required to verify whether the protective plate is damaged. The damage threshold test power is usually 1.5 times

 the power corresponding to the nominal OD value, with an irradiation time of 10 seconds, to observe whether there is ablation, discoloration, or cracks on the 

surface.

VIII. Result determination and annotation

Qualification criteria: The measured OD value should be greater than or equal to the nominal OD value. The report should specify the test wavelength, laser mode

 (continuous/pulsed), pulse width (if applicable), test spot diameter, and incident angle (usually 0° vertical incidence). If the incident angle is not vertical, it needs 

to be specifically stated in the report.

IX. Requirements for regular retesting

The laser protective plate should be retested at least once a year. If there is surface wear, aging, chemical contamination, or exposure to high temperature 

environments, it needs to be retested immediately. Products that fail the retest should be scrapped and not used further.

X. Safety precautions

During the testing process, operators must wear protective glasses with a level not lower than that of the testing laser. For high-power testing, it is necessary to set

 up a laser interlock device and an emergency stop button. A beam absorber should be installed at the end of the transmission light path to prevent secondary hazards

 caused by reflected light.