We have measured the quantum yield Q_Δ of ¹Δ_g molecular oxygen (singlet oxygen, ¹O₂) formation photosensitized by some dyes for industrial uses, because the photofading processes of dyes are usually triggered by ¹O₂. Q_Δ for industrial dyes, however, is generally very low, and it has been difficult to determine accurately Q_Δ for them. We have made a highly sensitive spectrometer for 1.27-μm ¹O₂ phosphorescence detection, and determined Q_Δ for an azo dye as low as (2.2 ± 1.5) × 10⁻⁵. This is the first demonstration of Q_Δ lower than 10⁻⁴ as far as we know. Relationship between Q_Δ and the lightfastness of practical dyes is also discussed in this study.