def calculate_trans(thickness_cm: np.float, miu_per_cm: np.array):
    """calculate the transmission signal using the formula

    transmission = exp( - thickness_cm * atoms_per_cm3 * 1e-24 * sigma_b)

    Parameters:
    ===========
    thickness: float (in cm)
    atoms_per_cm3: float (number of atoms per cm3 of element/isotope)
    sigma_b: np.array of sigma retrieved from database

    Returns:
    ========
    transmission array
    """
    transmission = np.exp(-thickness_cm * miu_per_cm)
    return np.array(transmission)