References designed specifically to check absorbance accuracy and linearity in the visible region are available from Starna. These references can be manufactured from optical glass rather than the UV grade quartz required for UV references.
Many Starna absorbance references designed principally for use in the UV also have useful peaks in the visible region, and are also listed below.
Neutral Density Filters (440 - 635nm)
These references can be used to qualify the absorbance accuracy and linearity of visible spectrophotometers with a spectral bandwidth of 6.5 nm or less, over the wavelength range 440 nm – 635 nm. 18 individual filters cover absorbance values from 0.4 A to 3.5A and convenient sets are also available. This reference is accepted for instrument qualification by a number of standardisation bodies, including:
United States Pharmacopoeia
American Society for Testing and Materials (ASTM)
Therapeutic Goods Administration (Australia)
Metal-on-Quartz Neutral Density Filters (250 - 635 nm)
These filters are equivalent to NIST SRM 2031. Absorbance values are certified at 250, 280, 340, 360, 400, 465, 500, 546.1, 590 and 635 nm. Approximate values available are:
90% T (0.04 A), 50% T (0.3 A), 30% T (0.5 A), 10% T (1.0 A), 3%T (1.5 A) and 1% T (2.0A). This type of filter can also be supplied with certified values in the NIR, up to 2500 nm.
Before purchase, users should check that these filters are compatible with their spectrophotometer - see data sheet.
Starna Green Broadband References (250 - 650 nm)
Specially developed for instruments with wider Spectral Band Widths (up to 20 nm), but a good general purpose reference, Starna Green has broad peaks at 258 nm, 416 nm and 630 nm. Four different concentrations cover an absorbance range from approximately 0.25 A to 1.A and the references are available singly or in any combination. This reference material may also be used to qualify the wavelength scale.
Combined Holmium/Neutral Density Glass
With this combined filter, the holmium spectrum is superimposed on an approximately constant 1A background, useful as an absorbance accuracy check. 7 peak wavelengths are certified at nominal values of 361, 419, 446, 454, 460, 537 and 638 nm.
Absorbance values, nominally 1A, are certified at wavelengths between the holmium peaks at 431, 516, 557, 594 and 623 nm.
Combined Didymium/Neutral Density Glass
With this combined filter, the holmium spectrum is superimposed on an approximately constant 1A background, useful as an absorbance accuracy check. 9 peak wavelengths are certified at nominal values of 473, 514, 530, 572, 586, 739, 748, 807 and 879 nm.
Absorbance values, nominally 1A, are certified at wavelengths between the didymium peaks, at 410, 455, 550, 637 and 773 nm.