Why does R correspond to Giant Pulsed lasers and D correspond to continuous wave?
Why did they split up the classes of lasers the way they did?

I searched the internet for the answers to these questions, but found only what the acronyms were and what laser classifications were.  The thought process behind them is strangely absent.

The following is my best guess at that thought process. Spoiler, the acronyms come from German, and the laser safety classification comes from which processes would save your eye from laser exposure.

Disclaimer

This is not safety advice. This is just my guess at Etymology.
Not responsible for anything you do with this info.

Rough naming convention for laser classifications

Lasers and Laser Products are both classified. For example, a Class 4 laser enclosed super well could theoretically be a Class 1 laser product.

• Class 1 = you “can” supposedly stare into the beam
  • • < 0.4 mW
• Class 2 = 0.25 s exposure is ok, so your blink reflexes will hopefully save you.
  • • <1 mW for visible light (blink response doesn’t work if invisible)
  • 2M = magnification of the beam is dangerous. it’s more power than the usual class 2, but it’s a large enough beam that it’s safe, while unmagnified.
• Class 3 = Blink response not good enough.
  • 3R = “regular” = highest power laser pointer you can buy.
    • <5mW
      • Supposedly only a “low hazard”, because the unsteadiness of pointing a hand held laser pointer will make exposure to the beam short. However, desk mounted 3R lasers will be pointed very stably, and hence still dangerous.
  • 3B = dangerous to the eye. The letter “B” meant that it was more dangerous than a 3A laser, which was latter deprecated.
    • <0.5W
• Class 4 = even the diffuse reflection off a piece of paper is dangerous
  • Fire hazard
  • Highest laser class.

Note, power levels for each class will change for different wavelengths, for different standards, and for non continuous-wave lasers.

These numbers are rounded from: Laser classification table – Laser Safety Facts

Laser Safety Glasses Standard

ANSI Z136 and EN207  = Laser Protection
EN208 = Laser alignment

Light Transmission

All of them have a # corresponding to transmission of the lens for each range of wavelengths.
Transmission = 10^-#

EN208 demarks this number as “R#”,
EN207 demarks this number as “L#” (e.g L4),
whereas ANSI writes “OD #”
My guess is that L means “level” or “log transmission”. OD means optical density. “R”, I have no idea where that comes from, maybe “reduction”.

Laser Type Rating

EN207 also has a few letters before the L# rating, which confused me, until I found out they came from German. These correspond to the type of laser the glasses are made for:

• D = “Dauerstrich” = Continuous Wave
• I = “Impuls” = Intermittent = pulsed laser
• R = “Riesenpulslaser” = Repetitive pulsed = Giant pulsed laser
• M = “modengekoppelter Laser” or “modengebundener Laser” = Mode locked

These letters are important, because high pulse power can saturate the absorption of laser glasses. The shorter the pulse, the higher the power. These pulse durations are listed on thorlabs.