def beam_area_pix(head):
    """
    Computes the beam area in pixels.
    It uses an approximation accurate to
    within 5%.
    
    K. Rohlfs and T.L. Wilson, 'Tools of Radio Astronomy', third revised and enlarged edition, 1996, Springer, page 190-191.
    
    :param head: Image header.
    :type head: Fits header
    :returns: Number of pixels inside the beam.
    :rtype: float
    """
    
    return 1.133*float(head['BMAJ'])*float(head['BMIN'])/(abs(head['CDELT1'])*abs(head['CDELT2']))