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  • Neutral Density Filters

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Starna Scientific Ltd.
52/54 Fowler Road
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Last updated 6 June 2018

 

Technique


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Visible References


  • Absorbance
    • Neutral Density Filters
    • Metal-on-Quartz Neutral Density Filters (250-635 nm)
    • Starna Green (Broadband SBW instruments) (250-650 nm)
    • Combined Holmium/Neutral Density glass (360 - 640 nm)
    • Combined Didymium/Neutral Density glass (430 - 890 nm)
  • Wavelength
    • Holmium Oxide Liquid (240-650 nm)
    • Didymium Oxide Liquid (290-870 nm)
    • Holmium Oxide Glass (270-640 nm)
    • Didymium Glass (430 nm to 890 nm)
    • Combined Holmium/Neutral Density glass (360 - 640 nm)
    • Combined Didymium/Neutral Density glass (430 - 890 nm)
    • Starna Green (Broadband SBW instruments) (250-650 nm)
    • Samarium Oxide Liquid (230-560 nm)

Neutral Density Filters

Purpose

Purpose

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 - 635 nm. They are accepted for this purpose by most pharmacopoeias and other regulatory bodies

Description and Discussion

Description and Discussion

Schott NG type glass filters, from which a wide range of transmission and absorbance can be produced, have been available for over forty years. All Starna neutral density filters are manufactured and certified in accordance with the ‘Technical Specification for Certification of Spectrophotometric NIST Traceable Reference Materials’ (NIST Special Publication 260-140). This recommends calibration at 440 nm, 465 nm 546.1 nm, 590 nm and 635 nm, and Starna filters are certified at these wavelengths. Starna absorbance references are calibrated at a bandwidth of 1 nm, however other bandwidths are available on request They can also be certified at other wavelengths to suit customer requirements with the provision that the 440 nm, 546.1 nm and 635 nm wavelengths are included for traceability.

A range of 18 filters is available from Starna, covering absorbance values from approximately 0.04 A to 3.5 A (see How to Order, below). Originally, these filters were manufactured with selected transmission values. Available transmission values and equivalent absorbances are:

% Transmission

Absorbance

92

0.04

79.4

0.10
73  0.14
60  0.22
56.5 0.25
50  0.30
40 0.40
30 0.52
25 0.60
20 0.70
10 1.00
6 1.22
3 1.52
1.5 1.82
1 2.00
0.3 2.52
0.1 3.00
0.06 3.22
0.03 3.52

This recommends calibration at 440 nm, 465 nm 546.1 nm 590 nm and 635 nm, and Starna filters are certified at these wavelengths.

NIST also lists three specifications for neutral density filter Standard Reference Materials (SRMs): SRM 930e, SRM 1930 and SRM 2930. These specifications also stipulate the maximum spectral bandwidth to be used at these wavelengths for certification measurements:

Wavelength(nm)

440

465

546.1

590

635

SBW (NM)

2.2

2.7

6.5

5.4

6.0

For those users wishing to work to these specifications, sets of filters equivalent to these SRMs are available (see How to Order). For convenience, but to comply with the SRM specifications, all certification measurements are made at a SBW of 1.0 nm. These sets consist of three filters together with an empty aluminium holder, to be used as a blank.

 

SRM 930e

SRM 1930

SRM 2930

 

%T

A

%T

A

%T A

Filter 1

10

1.0

1

2.0

0.1 3.0

Filter 2

20

0.7

3

1.5

0.3 2.5

Filter 3

30

0.5

50

0.3

92 0.04

These SRMs were originally specified in transmittance, the specified nominal transmittance and approximate absorbance values are given below: Certificate values are given in both %T and Absorbance.

 

Typical spectra

Typical spectra

Neutral-Density-Filters-

 
Catalogue Number

Catalogue Number

Individual filters and Starna selected sets:

These sets allow absorbance linearity qualification over a range of values.

Nominal % Transmittance Nominal Absorbance Catalogue Number
           
    Set of 4 filters Set of 6 filters Set of 9 filters  Single filter
    RM-4ND RM-6ND RM-9ND  
92 0.04     • RM-9N
 79.4 0.1       RM-8N
73 0.14     • RM-7N
60 0.22     • RM-6N
56.5 0.25 • •   RM-5D
50 0.3       RM-5N
40 0.4       RM-4N
30 0.52 • • • RM-3N
25 0.6       RM-2D
20 0.7       RM-2N
10 1 • • • RM-1N
6 1.22       RM-N6
3 1.52   • • RM-N3
1.5 1.82       RM-1D
1 2 • • • RM-N1
0.3 2.52     • RM-D3
0.1 3   • • RM-D1
0.06 3.22       RM-H6
0.03 3.52 RM-H3
0.01 4.00 RM-A1

Any combination of references can be ordered as a set by combining the single filter catalogue numbers.

NIST Equivalent SRM Sets

Three specifications for neutral density filter Standard Reference Materials (SRMs) are listed by NIST: SRM 930e, SRM 1930 and SRM 2930. For those users wishing to work to these specifications, sets of filters equivalent to these SRMs are available:

  Catalogue  Number

SRM 930e equivalent set:
10, 20 & 30 %T (1.0, 0,7 & 0.5A) and blank holder

RM-1N2N3N

SRM 1930 equivalent set
1, 3 & 50 %T (2.0, 1.5 & 0.3 A) and blank holder

RM-N1N35N

SRM 2930 equivalent set
0.1, 0.3 & 92 %T (3.0, 2.5 & 0.04 A) and blank holder 

RM-D1D39N

NIST SRM Set
Set of all 9 above filters and blank holder 

RM-SRM9ND

Visible Spectrophotometer Qualification Set

This is a convenient way of qualifying a visible spectrophotometer for photometric accuracy, linearity and wavelength accuracy.

  Catalogue  Number

Starna Transmission/Absorbance and Wavelength Set
Neutral Density Filters: 1.00 ,0.50 &0.25 A (10, 30 & 52 %T) and blank holder &

Holmium Glass Filter

1N3N5DHG

Product enquiry

Product enquiry

 

Data sheet

Data sheet

 

 

 

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