secondary spectrum doublet
第二级光谱双线

• null corrector
• null-indicatiing bouble-beam system
• null indicator
• nulling
• nulling optics
• nullity
• null lens
• null matrix
• null method
• null method of neasurement
• null offset
• null point
• null-reading
• null spectrophotometer
• null-tyjpe infrared spectrophotometer
• nulvalent
• number
• numberator
• numberical
• number of itnerference fringes
• numerical aperture
• numerical coding
• numerical control
• numerical reader
• numerical simulation