Let $I_{b\lambda }$ be the spectral blackbody radiation intensity per unit wavelength about the wavelength $\lambda$ .

The blackbody radiation intensity emitted by a blackbody over all wavelength is

1. $\frac{dI_{b\lambda }}{d\lambda }$
2. $\frac{d^{2}I_{b\lambda }}{d\lambda^{2} }$
3. $\int _{0}^{\infty }I_{b\lambda }d\lambda$
4. $\int _{0}^{\infty }\lambda I_{b\lambda }d\lambda$