Concave Gratings Rowland Type

Standard products

Information about our Concave Gratings...

When ordering a grating, please use the following example format:

C 1800 Dia 63.5xThk12.5 190-400 (TM+TE)/2 (-1) deviation angle 30°, RoC 500 mm

1.	C stands for Concave
2.	1800 is the groove density (groove frequency) in grooves/mm
3.	W is the blank dimension in mm parallell with the grating grooves
4.	H is the blank dimension in mm perpendicular to the grating grooves
5.	Dia is the diameter in mm
6.	Thk is the blank centre thickness in mm
7.	190-400 nm is the desired optimization range. A specific wavelength or range with peaked wavelength can also be specified
8.	(TM+TE)/2 (-1) is desired polarization state and diffraction order the grating should be optimized for. TE or TM can also be specified
9.	Constant deviation angle 30° is the configuration the grating should be optimized for. Constant incidence angle ° can also be specified

Standard tolerances on W, H, Dia: ± 0.2 mm Thk ± 0.5 mm
CA > 90 % of blank size

Standard concave gratings type C, specified by combining from:

Grooves/mm	Wavelength range:	Concave Radius:	Sizes:
0300 0600 1200 1800 2400 3600	UV 190-400 nm UV-VIS VIS 400-750 nm VIS-NIR NIR 750-1600 nm	500.0 400.0 750.1 998.8 2000.0	50 x 50 x 10 mm dia 50 x 12.5 mm dia 63.5 x 12.5 mm dia 63.5 x 12.5 mm dia 63.5 x 12.5 mm

Other specifications available on request, contact our sales department!

Concave Gratings Rowland Type

This type of grating is the holographic counterpart of the classically ruled concave grating, invented by Rowland in the 19th century. The Rowland grating has straight grooves which are equally spaced along a chord of the concave surface.

Rowland circle

These gratings are preferably used in mountings based on the Rowland circle, i.e. a circle with a diameter equal to the radius of curvature of the grating. If the entrance slit is located on the Rowland circle, then the spectral focus will also be on this circle.

If R is the radius of curvature, and a and b are the angles of incidence and diffraction, respectively, then

LA = Rcosa

LB = Rcosb

where LA and LB are the distances from grating centre to the entrance slit and spectral focus.

Good resolution

The aberrations (except astigmatism) are small for all points on the Rowland circle, making the gratings useful for wide spectral ranges.

Since all diffracted orders are focused on the Rowland circle, it is possible to make measurements on different orders simultaneously.

Extremely low stray light

Spectrogon's Rowland holographic gratings are fabricated with the same proprietary technique as our low stray light plane gratings.

Good efficiency

The sinusoidal groove profile of holographic gratings gives a better overall performance than a triangular "blazed" profile. Though a blazed concave grating may have higher efficiency at the grating centre, other points on the surface are "off blaze", giving lower efficiency.

Accurate groove frequency

The groove frequency (at the grating centre) is accurate within ±0.2 grooves/mm of the nominal value. The gratings are holographically exposed using optically flat wavefronts, ensuring straight and equispaced grooves.