Tubular daylighting devices remain a practical glass skylight option in 2026, especially where a full roof window is impractical. Their key measurement is diameter. Common sizes range from 250 to 550 millimeters. A 250 mm tube suits corridors, bathrooms, and compact utility rooms. It delivers useful daylight, but its bright area stays limited. A 350 or 400 mm tube generally offers a stronger balance between ceiling coverage and roof space. The 550 mm size can provide substantially higher light output for large rooms, although it needs careful structural planning.
Size alone does not determine performance. Tube length, roof angle, internal reflectance, diffuser design, and local sky conditions matter. A short, straight tube can transfer daylight efficiently. A long tube with several bends loses more light. In field measurements, two identical diameters may perform very differently under cloudy weather. The 550 mm model may produce broad illumination, not necessarily sharp brightness. That difference matters in kitchens, studios, and work areas.
My first estimate was too optimistic. I once judged output by diameter and ignored surrounding roof shade. That mistake changed the recommendation. For reliable design, compare tested illuminance data, not marketing language. Ask for results under clear and overcast conditions. Check the room’s floor area, ceiling height, and required task lighting. Smaller tubes can reduce installation disruption, but several units may create uneven patches. Larger tubes cost more and may still need electric lighting after sunset. Good specifications should acknowledge that limitation.