The nearly opaque B&W 105 mm Dark Red Infrared (092 = 89B) Filter, which looks dark purplish-red when held in front of a light source, blocks visible light up to 650 nm, and passes only 50% of the radiation just below 700 nm (thus the dark red color).
From 730 nm to 2000 nm, 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