Direct vs Diffused Viewing

In Laser Safety, there are several terms thrown around regarding types of viewing.  When reading laser safety classification, understanding proper laser safety, and the role PPE plays in laser safety, these are important terms to understand.   

The images below are merely illustration to depict the concept of direct viewing, specular viewing, and diffused viewing.  Intentional direct viewing or any type of direct viewing is not proper laser safety.  Other laser safety controls are designed to precent this.

Laser Safety Viewing: Direct, Specular, and Diffused
Intentional Direct Viewing (never do) Specular Viewing Diffused viewing or Reflection off non-reflective materials

Direct or Intrabeam viewing - 
The skin or eye is directly exposed to all or part of the laser.  An incidental direct viewing has an assumed time exposure of 0.25s; the reflex of the eye if hit to react (for visible lasers).  Please see below for assumed or anticipated viewing for an unintentional direct hit.  You should never look directly at a laser, and if hit, always look away immediately. The image above (to the left) illustrates an extreme example of intentional direct viewing - never do this.  

If you are to do intentional direct viewing, an independent calculation (beyond the other appropriate calculations for MPE) needs to be done to factor in the expected time duration, and then resultantly determine the needed optical density ("OD") to ensure you are below the Maximum Permissible Exposure or "MPE".  Often times, professional labs have a higher threshold in comparison to the maximum permissible exposure, for instance a threshold of 20% of the MPE. 

Specular viewing - which can be intrabeam/direct - Comes from reflections off smooth or mirrored surfaces. For instance, it can come from jewelry (which is why all jewelry should be removed from individuals).  Specular viewing can be as dangerous as direct or intrabeam viewing.

Diffused viewing or reflection - This is the reflection of rough surfaces, for instances a wood a table. This is typically the least dangerous type of viewing, however it should be noted that for class 4 lasers, diffused reflection can still cause fires.  Please note, the image is merely to display the concept of diffused viewing, do not get this close to a laser or its reflection from any surface (even with proper PPE).  Diffused viewing can cause permanent biological harm - always be sure to check with your laser safety officer before using a laser.

For an unintentional direct hit, the maximum anticipated viewing duration for proper laser safety typically falls into one of three categories:

a) Ultra-violet lasers (180-400nm) | assume 100 seconds
b) visible light lasers (400-700nm) | assuming 0.25 seconds
c) near infrared lasers (700-1000nm) | assume 10 seconds
d) For all other laser, use 600 seconds or laser time that is on up to 8 hours