Which statement about luminous infrared radiation sources is true?

• A. They never use tungsten filaments
• B. They always require a wavelength range outside 1500-12,000 mu
• C. They cannot penetrate tissue
• D. They may use tungsten filaments as a source

Answer: (D)

Luminous infrared radiation sources rely on radiating heat from a hot element, and tungsten filaments are a classic way to achieve that. When a tungsten filament is heated to high temperatures, it glows and emits a broad spectrum that includes substantial infrared energy. This makes tungsten filaments a common and practical source for infrared illumination, so the statement that they may use tungsten filaments as a source is true.

In contrast, saying they never use tungsten filaments isn’t accurate because these filaments are a traditional infrared source. The idea that they must operate only outside a specific long-wavelength band isn’t correct, since infrared devices can emit across different IR ranges, including wavelengths well within typical infrared applications. And dispensing with tissue penetration isn’t correct either, because infrared radiation can penetrate biological tissue to varying depths depending on wavelength.