import math

def calculate_radiant_intensity_precise(power_mW, aperture_diameter_m, distance_m):
    # Convert power from milliwatts to watts
    power_W = power_mW / 1000

    # Calculate the apex angle of the cone
    theta_rad = 2 * math.atan(aperture_diameter_m / (2 * distance_m))

    # Calculate the solid angle in steradians using the formula for a cone
    solid_angle_sr = 2 * math.pi * (1 - math.cos(theta_rad / 2))

    # Calculate radiant intensity in watts per steradian
    radiant_intensity_W_sr = power_W / solid_angle_sr

    # Convert radiant intensity from W/sr to mW/sr
    radiant_intensity_mW_sr = radiant_intensity_W_sr * 1000

    return radiant_intensity_mW_sr

# Example usage:
power = 200/1000 # in milliwatts
aperture_diameter = 0.009  # in meters (9mm)
distance = 0.01  # in meters (10mm)
print("Radiant Intensity (mW/sr):", calculate_radiant_intensity_precise(power, aperture_diameter, distance))