Spectral measure of the multiplication operator Let $(X,\mathcal B,\mu)$ be a finite measure space and consider the operator $T\colon L^2(X,\mu)\to L^2(X,\mu)$ given by $Tf(x)=\varphi(x)f(x)$, where $\varphi\colon X\to\mathbb R$ is a bounded measurable function. 
Is there any possibility to determine the spectral measure explicitly? 
 A: Put for $M\in\mathcal B$, where $\mathcal B$ is the $\sigma$-algebra on $X$, $E(M)=A_{\mathbf 1_M}$, where, for $g\in L^{\infty}(\mu)$, $A_g\colon L^2\to L^2$ $A_g(f)=fg$. $E$ is a spectral measure, since 


*

*$E(M)$ is a projection for all $M\in\mathcal B$: $$E(M)(E(M)g)(f)=E(M)(\mathbf 1_Mf)=\mathbf 1_M\cdot \mathbf 1_M \cdot f=E(M)f;$$

*$E(\emptyset)=0, $E(X)=Id$; 

*If $M$ and $N$ are disjoint then $E(M)$ and $E(N)$ are orthogonal, since $$E(M)(E(N)f)=\mathbf 1_M\mathbf 1_N f=0=E(N)(E(M)f).$$

*If $\{A_n\}\subset \mathcal B$ are disjoint then $$E\left(\bigcup_{n\in\mathbb N}A_n\right)(f)=\mathbf 1_{\bigcup_nA_n}f=\sum_{n=0}^{+\infty}\mathbf 1_{A_n}f=\sum_{n\in\mathbb N}E(A_n)(f).$$


Now check that $\int_f dE=A_f$ for all $f\in L^{\infty}$. $E$ is called the standard spectral measure.
A: Consider the space $B([a,b])$ of bounded Borel functions. We will endow it locally convex topology. Let $M([a,b])$ be a Banach space of complex-valued Borel measures on $[a,b]$. For each $\mu\in M([a,b])$ we define a semi-norm
$$
\Vert\cdot\Vert_\mu:B([a,b])\to\mathbb{R}_+: f\mapsto \int\limits_{[a,b]}f(t)d\mu(t).
$$
The family $\{\Vert\cdot\Vert_\mu:\mu\in M([a,b])\}$ gives rise to some Hausdorff locally convex topology on $B([a,b])$. We will denote this space $(B([a,b]),wm)$.Consider $\mathcal{B}(H)$ with weak operator topology and denote this space $(\mathcal{B}(H),wo)$. 
A continuous $*$-homomorphism $\gamma_{b,T}:(B([a,b]),wm)\to(\mathcal{B}(H),wo)$ such that $\gamma_{b,T}(id_{[a,b]})=T$ is called Borel functional calculus of operator $T$. 

Theorem
  There exist unique Borel functional calculus $\gamma_{b,T}:(B([a,b]),wm)\to(\mathcal{B}(H),wo)$. Moreover this is contractive involutive homomorphism of involutive algebras which extends continuous calculus on $[a,b]$.

Denote $H=L^2(X,\mu)$. Since $\varphi\colon X\to\mathbb{R}$ is a bounded measurable function then $T$ is a bounded selfadjoint operator. Hence $\sigma(T)\subset\mathbb{R}$. Since $T$ is selfadjoint then there exist some interval $[a,b]$ such that $\sigma(T)\subset[a,b]$. Now let $\gamma_{b,T}$ be the Borel functional calculus on $[a,b]$ then we can define spectral measure by the following procedure. For each Borel set $A\subset [a,b]$ we define its spectral measure $E(A)$ by equality 
$$
E(A)=\gamma_{b,T}(1_{A\cap[a,b]}).
$$
where $1_{A\cap[a,b]}$ is a characteristic function of $A\cap[a,b]$.
A: The spectral measure of $T$ is the measure $E(\Delta)=1_{\Delta}(T)$, where $\Delta$ is any Borel set in the spectrum of $T$, and $1_\Delta$ is the characteristic function of $\Delta$.
In your case, $T=M_\varphi $. Using functional calculus, we have, for any bounded Borel function $g$ on $\sigma(T)=\text {ess ran}\,\varphi$, that $g(T)\,f=(g\circ\varphi)\,f$ for any $f\in L^2(X,\mu)$. Then
$$
E(\Delta)f=1_\Delta(T)\,f=(1_\Delta\circ\varphi)\,f=1_{\varphi^{-1}(\Delta)}\,f.
$$
So, for each Borel set $\Delta$, the projection $E(\Delta)$ is the multiplication operator by the function $1_{\varphi^{-1}(\Delta)}$.
