EMIR is a multiobject intermediate resolution (R approximate to 4000) near infrared spectrograph with image capabilities to be mounted on the Gran Telescopio Canarias (GTC). EMIR shall provide image and spectra of a wide FOV (6x6 arcmin in imaging mode and 6x4 arcmin in multiobject spectroscopic mode), and will use grisms as dispersive elements. The use of grisms has great advantages in the design and manufacture of infrared spectrographs but there are not many suitable materials for the EMIR requirements. The grisms material must have good transmission in the working spectral range (1.0 - 2.5 mum) and given the required resolution, a high refractive index is necessary. Also the required homogeneity of the grism material makes it difficult to find a good candidate due to the large size of the EMIR grisms. process on large surfaces is an important issue to be Furthermore the technical difficulties related to the grooving addressed. Taking into account all those constraints and the EMIR requirements. several sets of materials, rulings and dimensions have been identified. These alternative solutions for EMIR grisms are proposed and analysed in terms of their physical characteristics, expected resolution, spectral coverage on detector and diffraction efficiency. Current status of the procurement of the devices will be given.
