The B&W Bay 2 Infrared Dark Red (092 = 89B) Filter is nearly opaque. It looks dark purplish/red when held in front of a light source and blocks visible light up to 650nm. It passes only 50% of the radiation just below 700nm (thus the dark red color).
This filter is specific to lenses with Rollei's exclusive Bay 2 filter size.
From 730 to 2000nm, transmission is greater than 90%; this makes photographs of pure red and infrared images possible with the best utilization of the relatively low sensitivity of infrared films
As the sensitization of infrared black and white films barely extends beyond 1000 nm, the red portion that is transmitted still makes a relevant contribution to the exposure; that is why this filter is the preferred filter for pictorial photography on IR black and white film